Fix that WALBuffer waits for flush instead of file-roll

[jt2594838]

Summary

• Bug: WALNode.ReqIterator used waitForFlush (signaled on buffer sync) to wait for new WAL data, but data only becomes readable after a WAL file roll — not a buffer flush. This caused the iterator to wake up prematurely (on flush) and then loop/timeout, or miss data entirely.
• Fix: Introduced a dedicated rollLogWriterCondition in WALBuffer that is signaled only when rollLogWriter creates a new WAL file. Renamed waitForFlush → waitForRollFile across IWALBuffer/WALBuffer/WALNode to reflect the corrected semantics.
• Additional fix: In waitForNextReady(time, unit), changed timeout || !hasNext() to timeout && !hasNext() — previously a successful (non-timeout) wake-up could still throw TimeoutException if hasNext() happened to return false during a race.
• Added tryToCollectInsertNodeAndBumpIndex.run() before updating file index so the iterator processes entries from the current file before advancing.

Changed files

• IWALBuffer.java — renamed interface methods waitForFlush → waitForRollFile
• WALBuffer.java — added rollLogWriterCondition, signal it in rollLogWriter(), use it in the renamed waitForRollFile methods
• WALNode.java — updated all call sites, fixed timeout || !hasNext() → timeout && !hasNext(), reordered collect call

Test plan

• Added WALNodeWaitForRollFileTest with 5 tests covering:
  • Timeout when no data is available
  • Flush without roll does not wake the iterator
  • Explicit roll wakes the iterator
  • Concurrent roll wakes a waiting iterator
  • Auto-roll on WAL file size threshold
  • Auto-triggered roll after timeout expiry

[JackieTien97]

don't remove

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.24%. Comparing base (d4be5c8) to head (44d3b74).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...storageengine/dataregion/wal/buffer/WALBuffer.java	83.33%	1 Missing ⚠️
.../db/storageengine/dataregion/wal/node/WALNode.java	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17628      +/-   ##
============================================
+ Coverage     40.23%   40.24%   +0.01%     
  Complexity     2554     2554              
============================================
  Files          5177     5177              
  Lines        348880   348884       +4     
  Branches      44624    44624              
============================================
+ Hits         140363   140400      +37     
+ Misses       208517   208484      -33     

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