backendcluster: route PD and backend traffic through cluster DNS (#1107)

[ti-chi-bot]

This is an automated cherry-pick of #1107

What problem does this PR solve?

Issue Number: close #1101

• docs: add cluster-scoped multi-cluster runtime design #1102
• config: add backend cluster schema and multi-port listener config #1103
• backendcluster: add cluster manager and cluster-scoped topology runtime #1104
• metricsreader: move cluster-scoped metrics lifecycle into backend clusters #1105
• router: add port-based routing groups #1106
• backendcluster: route PD and backend traffic through cluster DNS #1107 <-

What is changed and how it works:

Add cluster-scoped DNS routing for backend-cluster traffic.

This PR makes each backend cluster use its configured ns-servers for:

• PD / etcd traffic
• infosync-related topology lookups
• backend HTTP / MySQL traffic created from the cluster runtime

If ns-servers is empty, the default system resolver is still used.

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.

None

[YangKeao]

/retest

[codecov-commenter]

Codecov Report

❌ Patch coverage is 48.70317% with 178 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-nextgen-202603@4004711). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/testkit/dns_server.go	0.00%	95 Missing ⚠️
pkg/util/etcd/etcd.go	57.57%	19 Missing and 9 partials ⚠️
pkg/util/netutil/dns.go	75.29%	15 Missing and 6 partials ⚠️
pkg/proxy/proxy.go	0.00%	7 Missing and 1 partial ⚠️
pkg/manager/backendcluster/cluster.go	56.25%	6 Missing and 1 partial ⚠️
pkg/manager/backendcluster/network_router.go	72.00%	5 Missing and 2 partials ⚠️
pkg/manager/namespace/manager.go	0.00%	5 Missing ⚠️
pkg/balance/observer/health_check.go	86.36%	1 Missing and 2 partials ⚠️
pkg/balance/router/router_static.go	0.00%	2 Missing ⚠️
pkg/manager/backendcluster/manager.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##             release-nextgen-202603    #1126   +/-   ##
=========================================================
  Coverage                          ?   67.00%           
=========================================================
  Files                             ?      149           
  Lines                             ?    15675           
  Branches                          ?        0           
=========================================================
  Hits                              ?    10503           
  Misses                            ?     4455           
  Partials                          ?      717           

Flag	Coverage Δ
unit	67.00% <48.70%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YangKeao

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [YangKeao]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-04-13 10:43:47.239300233 +0000 UTC m=+1385032.444660300: ☑️ agreed by YangKeao.