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]

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.

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