[Subscription] Fix WAL replay after leader failover

[Caideyipi]

Description

Problem

After an IoTConsensus leader failover, the new leader can replay WAL entries replicated from the former leader. These entries carry valid writer progress but keep searchIndex = -1 because they do not have a local search index.

Replay-start lookup treated an uncovered follower entry before the first locally indexed entry as non-replayable and returned LOCATE_MISS. This surfaced as subscription error 1904 when one DataNode went down in a replicated cluster.

Fix

• Use only non-negative local search indexes to choose the local replay cursor.
• Keep replicated follower entries visible to ProgressWALIterator; writer progress continues to deduplicate already committed entries.
• Preserve the recovery writer progress when the WAL contains only uncovered follower entries.
• Add regression coverage for follower entries before a local lower bound and for the follower-only boundary.

No client change is required for this case. With another provider available, the existing consumer poll path marks the failed provider unavailable and continues failover; an application may temporarily receive an empty poll result, but it does not need a special retry for error 1904.

Validation

• mvn spotless:apply -pl iotdb-core/datanode
• mvn test -pl iotdb-core/datanode "-Dtest=ConsensusPrefetchingQueueTest,ProgressWALIteratorTest,ConsensusPrefetchingQueueWalBackpressureTest"
  • 40 tests run, 0 failures, 0 errors, 1 existing conditional skip
• mvn checkstyle:check -pl iotdb-core/datanode
• git diff --check

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This PR has:

• been self-reviewed.
• added comments explaining the intent of the non-obvious replay behavior.
• added or updated unit tests to cover the changed code paths.

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Key changed/added classes

• ConsensusPrefetchingQueue
• ConsensusPrefetchingQueueTest
• ProgressWALIteratorTest