Fix SegmentPurger to prevent unintended star-tree modification during purge

[praveenc7]

Summary

• When PurgeTask rebuilds a segment (after purging compliance/late-event records), SegmentPurger creates a new segment via SegmentGeneratorConfig which unconditionally picks up starTreeIndexConfigs from the table config. This causes old segments to unexpectedly gain star-tree indexes during purge, even when enableDynamicStarTreeCreation is false.
• This change makes SegmentPurger.initSegmentGeneratorConfig() respect enableDynamicStarTreeCreation — if it's false (default), star-tree configs are cleared before segment generation, preventing unintended star-tree add/remove/modify during purge rebuilds.

#17821

Test plan

unit test

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.28%. Comparing base (3ca14f9) to head (bf933ee).

Files with missing lines	Patch %	Lines
.../org/apache/pinot/core/startree/StarTreeUtils.java	92.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17822      +/-   ##
============================================
+ Coverage     63.20%   63.28%   +0.08%     
  Complexity     1456     1456              
============================================
  Files          3188     3188              
  Lines        191736   191768      +32     
  Branches      29347    29354       +7     
============================================
+ Hits         121178   121365     +187     
+ Misses        61067    60908     -159     
- Partials       9491     9495       +4     

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	63.25% <93.93%> (+0.07%)	⬆️
java-21	63.19% <93.93%> (+0.01%)	⬆️
temurin	63.28% <93.93%> (+0.08%)	⬆️
unittests	63.28% <93.93%> (+0.08%)	⬆️
unittests1	55.60% <93.93%> (+0.04%)	⬆️
unittests2	34.17% <0.00%> (+0.04%)	⬆️

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