[server] Require new leader ack for leader-only rebalance

[swuferhong]

Purpose

Linked issue: close #3555

Leader-only rebalance previously completed once the current leader matched
the target leader after any NotifyLeaderAndIsr response for the bucket. Since
NotifyLeaderAndIsr is sent to multiple replicas, this could complete the task
before the new leader acknowledged the request.

Pass the bucket result and response server id into the completion check, and
only complete leader-only rebalance when the result succeeded and the response
came from the target new leader. Keep LeaderAndIsr change handling from
directly completing leader-only rebalance tasks.

Brief change log

Tests

API and Format

Documentation

[Copilot]

Pull request overview

This PR fixes the completion criteria for leader-only rebalance in the coordinator so that a rebalance task is only marked complete after the target new leader has successfully acknowledged NotifyLeaderAndIsr, preventing premature completion when other replicas respond first (Issue #3555).

Changes:

• Pass NotifyLeaderAndIsr per-bucket result + responseServerId into rebalance completion checks.
• Require leader-only rebalance completion to be gated by: result.succeeded(), response from newLeader, and currentLeader == newLeader.
• Add/extend coordinator tests to verify leader-only rebalance waits for the new leader’s successful response.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
fluss-server/src/main/java/org/apache/fluss/server/coordinator/CoordinatorEventProcessor.java	Tightens leader-only rebalance completion conditions using the responding server id and per-bucket success, and avoids completing leader-only rebalance from leader/isr-change events.
fluss-server/src/test/java/org/apache/fluss/server/coordinator/CoordinatorEventProcessorTest.java	Adds targeted tests to ensure leader-only rebalance doesn’t complete on successful responses from non-target replicas and validates the new completion predicate.

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[swuferhong]

@wuchong copilot's comments addressed. PTAL, thanks.