[server] Fix flaky RebalanceManagerITCase.testRebalanceWithRemoteLog

[Kaixuan-Duan]

Purpose

Linked issue: close #2974

Brief change log

• Use retry to wait until the in-memory manifest is ready before reading remoteLogSize, since waitUntilSomeLogSegmentsCopyToRemote only ensures ZK has the manifest handle but the in-memory RemoteLogTablet may not be updated yet by LogTieringTask.
• Change the assertion on remote log segment count after rebalance from isEqualTo to isGreaterThanOrEqualTo, since remoteLogSize is captured when async remote log copy may not have finished uploading all segments yet, so the new leader's manifest can contain more segments.

Tests

./mvnw -pl fluss-server -Dtest=RebalanceManagerITCase#testRebalanceWithRemoteLog -DfailIfNoTests=false verify

API and Format

Documentation