tici: ensure config rolling update applies and use DNS flash.service_addr when TiCI enabled

[3pointer]

Summary

This PR fixes two TiCI-related issues:

TiCI meta/worker config updates may not roll out to Pods even after ConfigMap changes.
When TiCI is enabled, TiFlash flash.service_addr could default to 0.0.0.0 (line 3930) instead of the headless DNS address required by TiCI reader.

Root Cause

TiCI StatefulSet reconcile path did not force rolling upgrade partition for pending updates, so Pod revision could remain old (currentRevision != updateRevision) with partition stuck.
TiFlash default flash.service_addr path did not switch to headless service DNS for TiCI-enabled clusters in all config builders.

Tests:

Add TestPrepareTiCIRollingUpgrade.
Update TiFlash service_addr tests to assert headless DNS for TiCI clusters.

[3pointer]

/rebuild

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/retest

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/rebuild

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/rebuild

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csuzhangxc

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-02-27 01:56:44.995818643 +0000 UTC m=+406477.510613262: ☑️ agreed by csuzhangxc.

[ti-chi-bot]

@3pointer: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-e2e-kind-across-kubernetes	d1ef512	link	unknown	/test pull-e2e-kind-across-kubernetes
pull-e2e-kind-tidbcluster	d1ef512	link	unknown	/test pull-e2e-kind-tidbcluster

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