balance, proxy: support evicting backends by config (#1116)

[ti-chi-bot]

This is an automated cherry-pick of #1116

What problem does this PR solve?

Issue Number: close #1115

Problem Summary:
When a backend is confirmed to fail, we need a way to evict it manually.

What is changed and how it works:

• add proxy.fail-backend-list and proxy.failover-timeout config
• stop routing new connections to failed backends and migrate existing ones away
• force close remaining connections after the failover timeout
• if no more available backends, ignore the list to be HA
• allow fail-backend-list to match by pod name or backend address

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

- Support evict failed backends by config

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: djshow832

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Details

Needs approval from an approver in each of these files:

• OWNERS [djshow832]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-04-20 06:59:45.796992034 +0000 UTC m=+1976391.002352081: ☑️ agreed by djshow832.

[djshow832]

/retest

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.02620% with 32 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-nextgen-202603@349d1f7). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/balance/factor/factor_balance.go	0.00%	13 Missing ⚠️
pkg/balance/policy/simple_policy.go	0.00%	7 Missing ⚠️
pkg/balance/router/group.go	96.61%	2 Missing and 2 partials ⚠️
pkg/balance/router/router.go	92.85%	2 Missing and 1 partial ⚠️
pkg/proxy/backend/backend_conn_mgr.go	81.25%	2 Missing and 1 partial ⚠️
lib/config/proxy.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##             release-nextgen-202603    #1131   +/-   ##
=========================================================
  Coverage                          ?   67.25%           
=========================================================
  Files                             ?      149           
  Lines                             ?    15983           
  Branches                          ?        0           
=========================================================
  Hits                              ?    10749           
  Misses                            ?     4504           
  Partials                          ?      730           

Flag	Coverage Δ
unit	67.25% <86.02%> (?)

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