fix(segment): close removed PinotDataBuffer in FilePerIndexDirectory

[dkranchii]

FilePerIndexDirectory.removeIndex() was dropping the buffer entry
from the cache map without calling close() on it. For mmap-backed
buffers (the default for offline segments), this means the kernel
mapping leaks for the lifetime of the JVM — the on-disk file does
get deleted, but the address-space mapping stays until the process
exits (or the Cleaner happens to fire, which we shouldn't rely on
for deterministic cleanup).

removeIndex() is hit during segment reloads, minion-driven segment
conversions, and the various *IndexHandler drop/replace flows, so on
long-running servers that churn through reloads this shows up as the
classic.

The class itself had a // TODO flagging this, so this PR just
finishes it.

Scope

Single file: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/store/FilePerIndexDirectory.java
~10 lines changed

No public API change, no behavior change for callers and removeIndex still returns void. It still cleans up the on-disk files exactly the same way, just additionally closes the cached buffer.

Testing

Existing FilePerIndexDirectoryTest still passes.
Manually verified the flow: with the fix, the PinotDataBuffer.close() on the removed entry is invoked once; without the fix it never was.

Fixes #18404

Release note

none (memory leak fix, no user-visible behavior change)

[dkranchii]

@xiangfu0 can you please review this pr. thanks

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.60%. Comparing base (80fb0e8) to head (44afbfa).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
...ent/local/segment/store/FilePerIndexDirectory.java	70.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18405      +/-   ##
============================================
+ Coverage     63.46%   63.60%   +0.14%     
- Complexity     1701     1717      +16     
============================================
  Files          3254     3252       -2     
  Lines        199104   199141      +37     
  Branches      30830    30876      +46     
============================================
+ Hits         126354   126668     +314     
+ Misses        62665    62400     -265     
+ Partials      10085    10073      -12     

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-21	63.60% <70.00%> (+0.14%)	⬆️
temurin	63.60% <70.00%> (+0.14%)	⬆️
unittests	63.60% <70.00%> (+0.14%)	⬆️
unittests1	55.68% <10.00%> (+0.26%)	⬆️
unittests2	34.91% <70.00%> (-0.07%)	⬇️

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[dkranchii]

@Jackie-Jiang updated the pr. can you please review. thanks