[Backport 2.x] [Bugfix] Fixes IRC NPE bug for timed-out cacheable queries (#15327)

[peteralfonsi]

Backports #15327 to 2.x.

Description

When a cacheable query times out, the IRC lets it enter the cache and then invalidates it in the same search thread. In 2.13, invalidation was changed to require a readerCacheKeyId string. invalidate() only got this if the DirectoryReader was an OpenSearchDirectoryReader and otherwise it was left null. When the reader was an ExitableDirectoryReader, this caused an NPE, which was returned to the user. It also means the timed-out query response incorrectly remained in the cache and could be returned the next time the user made the query.

Fix this by getting readerCacheKeyId from reader.getReaderCacheHelper() without checking which class reader is. This is safe, as it's the same method used in IndicesRequestCache.getOrCompute(). Now the user correctly sees the timed-out response and it is invalidated from the cache.

Tested in an IT and manually with the java debugger.

Related Issues

Resolves #15175

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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[github-actions]

❌ Gradle check result for 0a5b036: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 759c902: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.62%. Comparing base (4f97fd3) to head (759c902).
Report is 1 commits behind head on 2.x.

Files	Patch %	Lines
...va/org/opensearch/indices/IndicesRequestCache.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #15390      +/-   ##
============================================
+ Coverage     71.52%   71.62%   +0.10%     
- Complexity    63228    63257      +29     
============================================
  Files          5197     5197              
  Lines        296727   296726       -1     
  Branches      43179    43178       -1     
============================================
+ Hits         212221   212534     +313     
+ Misses        66696    66386     -310     
+ Partials      17810    17806       -4     

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