coordinator: preserve stop operator on repeated warnings (#6135)

[ti-chi-bot]

This is an automated cherry-pick of #6135

What problem does this PR solve?

Issue Number: close #6134

What is changed and how it works?

Introduce a stop operator kind to distinguish current-placement stops from stale-owner cleanup.

Repeated non-removing stop requests now reuse the existing current-placement operator, preserving its target node and maintainer epoch. Stale-owner cleanup operators are not reused when the current owner needs to be stopped. This prevents maintainer recovery from getting stuck after repeated warning events.

Check List

Tests

• Unit test
• Integration test

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Fix a changefeed stuck issue after downstream failures.

Summary by CodeRabbit

• Bug Fixes
  • Improved changefeed stop handling when ownership changes.
  • Prevented duplicate stop requests from replacing an existing operation unnecessarily.
  • Ensured cleanup requests for stale owners do not interfere with stops targeting the current owner.
  • Preserved the original target and maintainer epoch when repeated stop requests are received.

[ti-chi-bot]

@wk989898 This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

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