Replace ad-hoc timestamp index guard with dedicated staleness check in isSegmentStale

[shounakmk219]

Summary

• The previous fix (Fix false-positive "column deleted" stale check for timestamp-index derived columns #18294) added a narrow guard inside the "column deleted" branch to suppress false positives when a timestamp index is removed. This was incomplete for two reasons raised in the PR review:
  • Removing a timestamp index should mark the segment stale so SegmentPreProcessor can clean up the orphaned $col$GRANULARITY columns — the old guard silently skipped them.
  • Adding a new granularity or configuring a timestamp index for the first time was misreported as "column added" rather than a dedicated timestamp index reason. Existing derived columns in the segment could also trigger spurious "range index changed" verdicts on every reload.
• Replaces the guard with a dedicated feature-level check (following the StarTree and MultiColumnText pattern): before the per-column loop, compare the set of $col$GRANULARITY columns expected by the current table config against those present in the segment. Any mismatch returns "timestamp index changed".
• The generic "column added" check now excludes $col$GRANULARITY columns (covered by the dedicated check), and the per-column loop skips them entirely to prevent false positives on "column deleted" and "range index changed".

Test plan

• Updated 4 existing timestamp index tests to assert "timestamp index changed" with isStale() == true for the add/remove/granularity-added/granularity-removed cases.
• testTimestampIndexNoChange continues to assert not stale.
• ./mvnw -pl pinot-core -am -Dtest=BaseTableDataManagerNeedRefreshTest -Dsurefire.failIfNoSpecifiedTests=false test — all 42 tests pass.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.60%. Comparing base (a239cb3) to head (35d1442).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
.../pinot/core/data/manager/BaseTableDataManager.java	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18310      +/-   ##
============================================
- Coverage     63.61%   63.60%   -0.01%     
  Complexity     1659     1659              
============================================
  Files          3246     3246              
  Lines        197514   197522       +8     
  Branches      30580    30580              
============================================
- Hits         125644   125633      -11     
- Misses        61823    61840      +17     
- Partials      10047    10049       +2     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.58% <92.30%> (+<0.01%)	⬆️
java-21	63.57% <92.30%> (-0.01%)	⬇️
temurin	63.60% <92.30%> (-0.01%)	⬇️
unittests	63.60% <92.30%> (-0.01%)	⬇️
unittests1	55.57% <92.30%> (-0.01%)	⬇️
unittests2	35.04% <0.00%> (-0.02%)	⬇️

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