[Bugfix] Fix TieredSpilloverCache stats not adding correctly when shards are closed

[peteralfonsi]

Description

Fixes a bug where the total stats for the TieredSpilloverCache are decremented incorrectly when shards were closed. Misses and evictions from both the heap and disk tier were subtracted from the total, but this is incorrect. When the disk tier is enabled, only disk-tier misses and evictions should count towards the cache total, so only they should be subtracted. Adds UTs and ITs around this.

Also adds UT coverage for TieredSpilloverCacheStatsHolder, which was missing before.

Related Issues

Resolves #16559

Check List

• Functionality includes testing.
• [N/A] API changes companion pull request created, if applicable.
• [N/A] Public documentation issue/PR created, if applicable.

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❌ Gradle check result for c2a05d4: FAILURE

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❌ Gradle check result for f85b719: FAILURE

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❌ Gradle check result for a39d586: FAILURE

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❌ Gradle check result for 165850e: FAILURE

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❕ Gradle check result for dbd0947: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.18%. Comparing base (b1a7743) to head (2e8ffee).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...search/cache/common/tier/TieredSpilloverCache.java	75.00%	0 Missing and 1 partial ⚠️
...e/common/tier/TieredSpilloverCacheStatsHolder.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16560      +/-   ##
============================================
+ Coverage     72.16%   72.18%   +0.02%     
+ Complexity    65246    65225      -21     
============================================
  Files          5318     5318              
  Lines        303940   303945       +5     
  Branches      43976    43976              
============================================
+ Hits         219326   219412      +86     
+ Misses        66625    66520     -105     
- Partials      17989    18013      +24     

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[github-actions]

❌ Gradle check result for 0de6284:

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✅ Gradle check result for 2e8ffee: SUCCESS