Fix flaky WALNodeTest WAL roll assertion

[jt2594838]

Wait for all synchronous WAL flush listeners before asserting rolled WAL file names. This prevents the test from observing the buffer as idle before the sync task completes WAL rolling. Validation: targeted test passed, plus 10 consecutive repeated runs passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.78%. Comparing base (823c47e) to head (d0f1778).

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18226      +/-   ##
============================================
- Coverage     42.79%   42.78%   -0.01%     
  Complexity      343      343              
============================================
  Files          5356     5356              
  Lines        380294   380294              
  Branches      49332    49332              
============================================
- Hits         162730   162707      -23     
- Misses       217564   217587      +23     

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