test: refactor selfmonitor tests with shared helpers and HTTP 400/429 fault injection

[jeffreylimnardy]

Description

Changes proposed in this pull request (what was done and why):

• Refactor selfmonitor E2E tests to extract shared helper functions (flowHealthyThenDegraded, assertFlowDegraded, buildPipeline, defaultGenerator, assertComponentReady, assertPipelineHealthy) that eliminate repetition across outage, backpressure, and healthy test scenarios
• Replace Istio VirtualService fault injection with the mock-backend (merged in test: add mock-backend for lightweight fault injection in tests #3283) for most components: the mock-backend supports configurable HTTP status codes, delays, and connection-close behavior — used to simulate NoLogsDelivered (TCP close on port 9880) and BufferFillingUp (HTTP 429 retries + delayed 200 responses) without requiring Istio
• Metric-agent tests still use Istio VirtualService with source-label targeting (app.kubernetes.io/name: telemetry-metric-agent): the metric-agent and gateway share the same backend, so faulting it at the backend level would affect both legs; a source-label-scoped VirtualService selectively blocks only agent→gateway traffic while leaving the gateway's own exports healthy
• Replace HTTP 500 (which triggered retries) with HTTP 400 as the universal non-retryable fault code, and HTTP 429 as the universal retryable fault code, making test behavior more deterministic across OTel Collector and Fluent Bit
• Split fluent-bit backpressure into two test cases: fluent-bit-buffer-filling-up (429 retryable + delayed 200 to hold the queue full) and fluent-bit-data-dropped (400 non-retryable drop)
• Split fluent-bit outage into fluent-bit-no-logs-delivered (TCP close, simulating ECONNREFUSED-like behavior) and fluent-bit-all-data-dropped (100% HTTP 400)
• Add WithFluentBitHostPathCleanup to clear /var/telemetry-fluent-bit on nodes after Fluent Bit tests, preventing buffer state bleed between test runs
• Centralize FlowHealthConditionTransitionTimeout (10 min) in periodic.go to replace duplicated inline 10*time.Minute constants

Changes refer to particular issues, PRs or documents:

• No related issue; this is a test-infrastructure-only refactoring.
• Depends on mock-backend from test: add mock-backend for lightweight fault injection in tests #3283.

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested. — N/A (test-only change)
• The feature is e2e-tested. — N/A (this PR IS the e2e test change)