maintainer: make dispatcher operator admission atomic (#6070)

[ti-chi-bot]

This is an automated cherry-pick of #6070

What problem does this PR solve?

Issue Number: close #6069

What is changed and how it works?

This change prevents multiple operators for the same dispatcher from being admitted concurrently.

Previously, AddOperator checked whether an operator already existed under a read lock, released the lock, and registered the new operator later under a write lock. Two concurrent operations, such as an Add and a Move for the same dispatcher, could both pass the initial check. Both operators would then be started and placed in the running queue, even though one overwrote the other in the operator map. This could create the same dispatcher on different TiCDC nodes.

An empty-origin Move is still allowed. If an Add and an empty-origin Move race for the same dispatcher, atomic admission ensures that only one can win. If the Move wins, it proceeds to create the dispatcher only on its destination node.

The barrier handling path is also hardened to accept a dispatcher’s block status only when the reporting node is the dispatcher’s current owner. Statuses from stale or non-owner dispatcher instances are ignored, preventing them from advancing a DDL or sync-point barrier.

Now the stale merge rollback will no longer mistakenly cancel a later installed remove operator.

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 race condition that could create duplicate dispatchers and cause downstream data inconsistency.

Summary by CodeRabbit

Bug Fixes

• Block status updates from non-owner nodes are now safely ignored.
• Unknown dispatchers and invalid statuses no longer trigger barrier processing.
• Operator registration now safely rejects concurrent duplicate requests.
• Replica moves are supported when the original node is unspecified.
• Operator replacement is now serialized with normal admission, preventing conflicting operations.
• Stale rollback actions can no longer cancel a newer replacement operator.

[ti-chi-bot]

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[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]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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