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

[djshow832]

This is a manual 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]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
2. For pull requests to TiDB-x branches, it must have no failed tests.
3. AFTER it has lgtm and approved labels, please wait for the cherry-pick merging approval from triage owners.

Details

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.41667% with 35 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-1.3@90ecc64). 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/router.go	86.36%	4 Missing and 2 partials ⚠️
pkg/balance/router/router_score.go	97.10%	2 Missing and 2 partials ⚠️
pkg/proxy/backend/backend_conn_mgr.go	81.25%	2 Missing and 1 partial ⚠️
lib/config/proxy.go	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release-1.3    #1133   +/-   ##
==============================================
  Coverage               ?   65.44%           
==============================================
  Files                  ?      123           
  Lines                  ?    11947           
  Branches               ?        0           
==============================================
  Hits                   ?     7819           
  Misses                 ?     3603           
  Partials               ?      525           

Flag	Coverage Δ
unit	65.44% <85.41%> (?)

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[ti-chi-bot]

@YangKeao: adding LGTM is restricted to approvers and reviewers in OWNERS files.

Details

In response to this:

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[djshow832]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YangKeao

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Details

Needs approval from an approver in each of these files:

• OWNERS [YangKeao]

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

[LGTM Timeline notifier]

Timeline:

• 2026-04-21 08:12:29.930553689 +0000 UTC m=+2067155.135913746: ☑️ agreed by YangKeao.