[CONTP-849] Fix loop detection bug and add unit tests for cluster agent leader election

[zhuminyi]

What does this PR do?

• Add unit tests for cluster agent leader election and leadership transitions
• Fix bug in LeaderForwarder.Forward() where loop detection didn't return after writing error

Motivation

Fixed a bug in pkg/clusteragent/api/leader_forwarder.go where loop detection (via X-DCA-Follower-Forwarded header) wrote an HTTP 508 error but continued to forward the request to the leader anyway.
Added missing return statement to properly short-circuit the request.

New Tests
pkg/clusteragent/api/leader_forwarder_test.go

• TestLeaderForwarder_SetLeaderIP - tests leader IP setting, updating, and clearing
• TestLeaderForwarder_Forward_NilProxy - tests behavior when no leader is configured
• TestLeaderForwarder_Forward_LoopDetection - tests that forwarding loops are properly detected and rejected
• TestLeaderForwarder_Forward_WithLeader - tests successful forwarding to leader

pkg/clusteragent/api/leader_handler_test.go

• Enhanced fakeLeaderForwarder mock to track leader IP changes and forward calls
• TestRejectOrForwardLeaderQuery_LeadershipTransition - tests behavior when leadership changes (leader → follower → leader)
• TestRejectOrForwardLeaderQuery_LeaderIPChange - tests that forwarder is updated when leader IP changes during follower state

Describe how you validated your changes

Additional Notes

[cit-pr-commenter]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: 02f5ae9e-d525-4a23-ba30-5d0fa5dfec40

Baseline: 92bf342
Comparison: 0169589
Diff

Optimization Goals: ✅ No significant changes detected

Experiments ignored for regressions

Regressions in experiments with settings containing erratic: true are ignored.

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	docker_containers_cpu	% cpu utilization	-0.56	[-3.55, +2.43]	1	Logs

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	file_tree	memory utilization	+0.85	[+0.78, +0.92]	1	Logs
➖	quality_gate_idle	memory utilization	+0.56	[+0.51, +0.61]	1	Logs bounds checks dashboard
➖	docker_containers_memory	memory utilization	+0.51	[+0.43, +0.58]	1	Logs
➖	uds_dogstatsd_20mb_12k_contexts_20_senders	memory utilization	+0.43	[+0.38, +0.49]	1	Logs
➖	file_to_blackhole_1000ms_latency	egress throughput	+0.12	[-0.29, +0.52]	1	Logs
➖	ddot_metrics	memory utilization	+0.11	[-0.12, +0.34]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	+0.08	[-0.29, +0.45]	1	Logs
➖	ddot_logs	memory utilization	+0.07	[+0.00, +0.14]	1	Logs
➖	ddot_metrics_sum_cumulativetodelta_exporter	memory utilization	+0.06	[-0.17, +0.29]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	+0.03	[-0.02, +0.07]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	+0.00	[-0.08, +0.08]	1	Logs
➖	otlp_ingest_metrics	memory utilization	+0.00	[-0.15, +0.15]	1	Logs
➖	uds_dogstatsd_to_api_v3	ingress throughput	-0.00	[-0.13, +0.12]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	-0.01	[-0.14, +0.12]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	-0.05	[-0.43, +0.34]	1	Logs
➖	ddot_metrics_sum_cumulative	memory utilization	-0.15	[-0.31, +0.02]	1	Logs
➖	ddot_metrics_sum_delta	memory utilization	-0.17	[-0.38, +0.04]	1	Logs
➖	quality_gate_idle_all_features	memory utilization	-0.22	[-0.26, -0.19]	1	Logs bounds checks dashboard
➖	otlp_ingest_logs	memory utilization	-0.24	[-0.34, -0.14]	1	Logs
➖	quality_gate_metrics_logs	memory utilization	-0.43	[-0.64, -0.22]	1	Logs bounds checks dashboard
➖	docker_containers_cpu	% cpu utilization	-0.56	[-3.55, +2.43]	1	Logs
➖	quality_gate_logs	% cpu utilization	-1.30	[-2.75, +0.15]	1	Logs bounds checks dashboard
➖	tcp_syslog_to_blackhole	ingress throughput	-2.40	[-2.49, -2.32]	1	Logs

Bounds Checks: ✅ Passed

perf	experiment	bounds_check_name	replicates_passed	links
✅	docker_containers_cpu	simple_check_run	10/10
✅	docker_containers_memory	memory_usage	10/10
✅	docker_containers_memory	simple_check_run	10/10
✅	file_to_blackhole_0ms_latency	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency	memory_usage	10/10
✅	file_to_blackhole_1000ms_latency	lost_bytes	10/10
✅	file_to_blackhole_1000ms_latency	memory_usage	10/10
✅	file_to_blackhole_100ms_latency	lost_bytes	10/10
✅	file_to_blackhole_100ms_latency	memory_usage	10/10
✅	file_to_blackhole_500ms_latency	lost_bytes	10/10
✅	file_to_blackhole_500ms_latency	memory_usage	10/10
✅	quality_gate_idle	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_logs	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	cpu_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	memory_usage	10/10	bounds checks dashboard

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

CI Pass/Fail Decision

✅ Passed. All Quality Gates passed.

• quality_gate_metrics_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check cpu_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.

[agent-platform-auto-pr]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor f1a9f13

Successful checks

Info

	Quality gate	Delta	On disk size (MiB)	Delta	On wire size (MiB)
✅	agent_deb_amd64	$${+0.01}$$	$${705.43}$$ < $${706.32}$$	$${-0.02}$$	$${172.9}$$ < $${173.81}$$
✅	agent_deb_amd64_fips	$${+0}$$	$${700.76}$$ < $${701.16}$$	$${+0.05}$$	$${171.95}$$ < $${172.71}$$
✅	agent_heroku_amd64	$${+0.01}$$	$${327.44}$$ < $${327.69}$$	$${+0}$$	$${87.37}$$ < $${88.19}$$
✅	agent_msi	$${-0.1}$$	$${979.1}$$ < $${982.08}$$	$${+0.02}$$	$${141.85}$$ < $${143.01}$$
✅	agent_rpm_amd64	$${+0.01}$$	$${705.41}$$ < $${706.31}$$	$${-0.03}$$	$${175.96}$$ < $${176.86}$$
✅	agent_rpm_amd64_fips	$${+0}$$	$${700.74}$$ < $${701.15}$$	$${+0.02}$$	$${174.01}$$ < $${174.95}$$
✅	agent_rpm_arm64	$${+0.01}$$	$${691.08}$$ < $${691.67}$$	$${+0.01}$$	$${159.78}$$ < $${160.7}$$
✅	agent_rpm_arm64_fips	$${+0.01}$$	$${687.31}$$ < $${687.54}$$	$${+0.02}$$	$${159.29}$$ < $${160.1}$$
✅	agent_suse_amd64	$${+0.01}$$	$${705.41}$$ < $${706.31}$$	$${-0.03}$$	$${175.96}$$ < $${176.86}$$
✅	agent_suse_amd64_fips	$${+0}$$	$${700.74}$$ < $${701.15}$$	$${+0.02}$$	$${174.01}$$ < $${174.95}$$
✅	agent_suse_arm64	$${+0.01}$$	$${691.08}$$ < $${691.67}$$	$${+0.01}$$	$${159.78}$$ < $${160.7}$$
✅	agent_suse_arm64_fips	$${+0.01}$$	$${687.31}$$ < $${687.54}$$	$${+0.02}$$	$${159.29}$$ < $${160.1}$$
✅	docker_agent_amd64	$${+0.01}$$	$${767.74}$$ < $${768.66}$$	$${-0}$$	$${260.68}$$ < $${261.52}$$
✅	docker_agent_arm64	$${+0.01}$$	$${777.7}$$ < $${778.35}$$	$${+0.01}$$	$${250.86}$$ < $${251.72}$$
✅	docker_agent_jmx_amd64	$${+0.01}$$	$${958.62}$$ < $${959.54}$$	$${-0}$$	$${329.31}$$ < $${330.15}$$
✅	docker_agent_jmx_arm64	$${+0.01}$$	$${957.3}$$ < $${957.95}$$	$${+0.01}$$	$${315.49}$$ < $${316.35}$$
✅	docker_cluster_agent_amd64	$${-0}$$	$${178.89}$$ < $${179.28}$$	$${-0}$$	$${63.18}$$ < $${63.96}$$
✅	docker_cluster_agent_arm64	$${+0}$$	$${196.39}$$ < $${196.79}$$	$${-0}$$	$${59.9}$$ < $${60.69}$$
✅	docker_cws_instrumentation_amd64	$${0}$$	$${7.07}$$ < $${7.12}$$	$${+0}$$	$${2.95}$$ < $${3.29}$$
✅	docker_cws_instrumentation_arm64	$${-0}$$	$${6.69}$$ < $${6.92}$$	$${-0}$$	$${2.71}$$ < $${3.07}$$
✅	docker_dogstatsd_amd64	$${0}$$	$${38.66}$$ < $${39.3}$$	$${-0}$$	$${14.93}$$ < $${15.76}$$
✅	docker_dogstatsd_arm64	$${0}$$	$${37.3}$$ < $${37.94}$$	$${-0}$$	$${14.37}$$ < $${14.83}$$
✅	dogstatsd_deb_amd64	$${0}$$	$${29.88}$$ < $${30.53}$$	$${+0}$$	$${7.88}$$ < $${8.75}$$
✅	dogstatsd_deb_arm64	$${0}$$	$${28.39}$$ < $${29.11}$$	$${-0}$$	$${6.82}$$ < $${7.71}$$
✅	dogstatsd_rpm_amd64	$${0}$$	$${29.88}$$ < $${30.53}$$	$${-0}$$	$${7.89}$$ < $${8.76}$$
✅	dogstatsd_suse_amd64	$${0}$$	$${29.88}$$ < $${30.53}$$	$${-0}$$	$${7.89}$$ < $${8.76}$$
✅	iot_agent_deb_amd64	$${0}$$	$${42.61}$$ < $${43.19}$$	$${-0}$$	$${11.13}$$ < $${11.98}$$
✅	iot_agent_deb_arm64	$${0}$$	$${40.35}$$ < $${40.92}$$	$${+0}$$	$${9.58}$$ < $${10.45}$$
✅	iot_agent_deb_armhf	$${0}$$	$${40.25}$$ < $${40.86}$$	$${+0}$$	$${9.7}$$ < $${10.56}$$
✅	iot_agent_rpm_amd64	$${0}$$	$${42.61}$$ < $${43.19}$$	$${+0}$$	$${11.14}$$ < $${12.0}$$
✅	iot_agent_suse_amd64	$${0}$$	$${42.61}$$ < $${43.19}$$	$${+0}$$	$${11.14}$$ < $${12.0}$$

[zhuminyi]

/merge

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[zhuminyi]

/merge --cancel

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[zhuminyi]

/remove

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[gabedos]

loop detection tests look good to me

[zhuminyi]

/merge

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• protobuf_test

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[zhuminyi]

/merge

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