Update tests for backwards codecs

[jmazanec15]

Description

Updates tests for backwards codecs to prevent backwards codecs from writing with a read only codec.

Right now, testKnnVectorIndex requires that the Hnsw Lucene Format have the capability to write. For backwards codecs, however, they are not able to write (see https://github.com/apache/lucene/blob/main/lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene94/Lucene94HnswVectorsFormat.java#L150).

To resolve this, we are removing the "testKnnVectorIndex" from backwards compatibility tests. For reading/writing backwards compatibility testing, we will rely on Lucene, where tests can be found in https://github.com/apache/lucene/tree/main/lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene94.

It adds a test that checks that our custom format is correctly overriden.

Issues Resolved

https://build.ci.opensearch.org/job/distribution-build-opensearch/6804/

Check List

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[martin-gaievski]

Maybe use class of format class the is defined in the version enum, something like:

assertThat(codec.knnVectorsFormat(), isA(V_9_4_0.getPerFieldKnnVectorsFormat().getClass()));

[jmazanec15]

I believe that assertThat is deprecated

[martin-gaievski]

org.hamcrest.MatcherAssert.assertThat should be ok

[martin-gaievski]

same comment as above

[codecov-commenter]

Codecov Report

Merging #710 (ed0a642) into main (6bbf086) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #710      +/-   ##
============================================
+ Coverage     84.37%   84.39%   +0.02%     
- Complexity     1066     1067       +1     
============================================
  Files           151      151              
  Lines          4345     4345              
  Branches        389      389              
============================================
+ Hits           3666     3667       +1     
+ Misses          499      498       -1     
  Partials        180      180              

Impacted Files	Coverage Δ
...ec/KNN940Codec/KNN940PerFieldKnnVectorsFormat.java	50.00% <0.00%> (-25.00%)	⬇️
...earch/knn/index/codec/KNN920Codec/KNN920Codec.java	90.90% <0.00%> (+9.09%)	⬆️
...earch/knn/index/codec/KNN910Codec/KNN910Codec.java	100.00% <0.00%> (+12.50%)	⬆️

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-710-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 8b168311d345228c30408bbb8078943b13477823
# Push it to GitHub
git push --set-upstream origin backport/backport-710-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-710-to-2.x.