[FLUSS-2401] Fix race condition where rebalance completes before leaders migrate

[ZuebeyirEser]

Purpose

Linked issue: close #2405

Brief change log

This PR fixes a race condition where a rebalance task could be marked as COMPLETED before the old replicas were actually stopped and leaders were migrated. This was especially visible under high load (e.g., 400 buckets).

The root cause was that CoordinatorEventProcessor used a "fire-and-forget" approach for StopReplica requests. It manually moved replicas to the Successful state immediately after sending the RPC.

Major changes:

• Updated RebalanceManager to track pendingDeletions for each bucket.
• Changed CoordinatorEventProcessor to wait for the DeleteReplicaResponseReceivedEvent callback before marking a task as finished.
• Added logic to handle DeadTabletServerEvent so rebalance tasks don't hang if a server dies while waiting for a response.
• Improved RebalanceManager#buildClusterModel robustness to skip leaderless buckets (avoids crashes during table initialization).
• Correctly separated state transitions for rebalance replicas (immediate cleanup) and table-deletion replicas (cleanup via TableManager).

Tests

• Added RebalanceRaceConditionITCase (regression test with 50 tables/400 buckets).
• Updated RebalanceITCase with stricter leader-count assertions.
• Verified all CoordinatorEventProcessorTest cases pass.

API and Format

no

Documentation

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