[Subscription] Flush lingering batch before WAL gap retry

[Caideyipi]

Summary

• Flush materialized lingerBatch entries before returning from the pending replay WAL_GAP path.
• Preserve stale seek handling when flushing fails.
• Add regression coverage for pending WAL gap retries so already materialized tablets remain pollable.

Tests

• mvn -o -pl iotdb-core/datanode -Dtest=ConsensusPrefetchingQueueTest#testPendingWalGapFlushesLingerBatchBeforeRetry+testPendingGapReplayHonorsPerRoundWalEntryLimit+testWalReplayRetriesGapWithoutSkippingEntries -DfailIfNoTests=false test
• mvn -o -pl iotdb-core/datanode -Dtest=ConsensusPrefetchingQueueWalBackpressureTest -DfailIfNoTests=false test

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.54%. Comparing base (0c9fb83) to head (1e50448).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...on/broker/consensus/ConsensusPrefetchingQueue.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18785   +/-   ##
=========================================
  Coverage     45.54%   45.54%           
  Complexity      712      712           
=========================================
  Files          5483     5483           
  Lines        396324   396327    +3     
  Branches      51575    51576    +1     
=========================================
+ Hits         180505   180520   +15     
+ Misses       215819   215807   -12     

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