[Vector Upsert 2/5] Harden vector search metric recording and backend param cleanup

[xiangfu0]

Part 2/5 of the split of #19287 (Fix FULL-upsert vector candidate generation).

Summary

Two robustness fixes in VectorSimilarityFilterOperator.executeSearch:

• Only record a vector search metric when a search actually executed. Previously a
  failure inside configureBackendParams still recorded a search that never ran.
• Guarantee backend search parameters are cleared even when configuration fails partway
  through the setters or the explain-context refresh throws, by claiming cleanup before
  the first setter and clearing in a nested finally. Backend search parameters
  (nprobe, efSearch) are ThreadLocal overrides on shared per-segment readers, so a
  skipped cleanup leaks a stale override onto a pooled query thread where it silently
  applies to a later query's search.

Part 4/5 relies on this hardening for its early-return exact-scan path.

Validation

• VectorSimilarityFilterOperatorTest: 23 tests passed, including the new
  testBackendParamsAreClearedWhenConfigurationFailsPartway.

Stacked on #19297 — this PR shows its commits until that merges; review only the last commit here.

Stack

1. [Vector Upsert 1/5] Support allowed-document filtering in exact vector scan #19297 — exact-scan allowed-document filtering
2. [Vector Upsert 2/5] Harden vector search metric recording and backend param cleanup #19298 — backend param cleanup hardening
3. [Vector Upsert 3/5] Never materialize nested vector subtrees as metadata filters #19299 — nested vector metadata guard
4. [Vector Upsert 4/5] Apply the query's visible-document set before vector candidate generation #19300 — apply the visible-document set before candidate generation (core fix)
5. [Vector Upsert 5/5] Add FULL-upsert vector integration coverage #19301 — integration coverage

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.89474% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.21%. Comparing base (e4888b0) to head (5796c1c).

Files with missing lines	Patch %	Lines
...operator/filter/ExactVectorScanFilterOperator.java	83.05%	6 Missing and 4 partials ⚠️
...not/core/operator/filter/VectorCandidateScope.java	75.00%	2 Missing ⚠️
...perator/filter/VectorSimilarityFilterOperator.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19298      +/-   ##
============================================
+ Coverage     67.20%   67.21%   +0.01%     
  Complexity     1424     1424              
============================================
  Files          3462     3463       +1     
  Lines        220361   220394      +33     
  Branches      35147    35158      +11     
============================================
+ Hits         148087   148134      +47     
+ Misses        60467    60448      -19     
- Partials      11807    11812       +5     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.21% <82.89%> (+0.01%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.21% <82.89%> (+0.01%)	⬆️
unittests	67.21% <82.89%> (+0.01%)	⬆️
unittests1	57.84% <82.89%> (+<0.01%)	⬆️
unittests2	39.22% <0.00%> (+<0.01%)	⬆️

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