Add IGNORE NULLS support for LEAD and LAG window functions

[yashmayya]

• Replace custom PinotLeadWindowFunction / PinotLagWindowFunction (which blocked IGNORE NULLS at the Calcite level) with standard SqlStdOperatorTable.LEAD / SqlStdOperatorTable.LAG
• Add processRowsIgnoreNulls() to LeadValueWindowFunction and LagValueWindowFunction using a bounded ArrayDeque for O(N) time and O(offset) memory
• LEAD scans right-to-left and LAG scans left-to-right, each maintaining a sliding window of the last offset non-null values
• Support all three operand forms: LEAD/LAG(col), LEAD/LAG(col, offset), and LEAD/LAG(col, offset, default)
• Add 7 unit tests covering default offset, custom offset, offset with default value, all-nulls edge case, and multiple partitions
• Add 5 resource-based query tests including LEAD/LAG IGNORE NULLS with various configurations and an explicit RESPECT NULLS regression guard
• Add @Nullable annotation to WindowFunction.extractValueFromRow()
• Fix pre-existing Javadoc typo in LeadValueWindowFunction ("LAG" → "LEAD")

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.34%. Comparing base (d299729) to head (50c6b87).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18202      +/-   ##
============================================
+ Coverage     63.31%   63.34%   +0.03%     
  Complexity     1627     1627              
============================================
  Files          3229     3238       +9     
  Lines        196707   197020     +313     
  Branches      30408    30471      +63     
============================================
+ Hits         124538   124801     +263     
- Misses        62192    62219      +27     
- Partials       9977    10000      +23     

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	63.32% <100.00%> (+8.06%)	⬆️
java-21	63.32% <100.00%> (+0.02%)	⬆️
temurin	63.34% <100.00%> (+0.03%)	⬆️
unittests	63.34% <100.00%> (+0.03%)	⬆️
unittests1	55.29% <100.00%> (+<0.01%)	⬆️
unittests2	34.95% <0.00%> (-0.01%)	⬇️

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