Fix BuildScoreProvider::randomAccessScoreProvider bug

[michaeljmarshall]

See comment: https://github.com/datastax/jvector/pull/536/files#r2453090826

In #536, there was an addition that prevented concurrent addition of vectors to the RandomAccessVectorValues object. This behavior has been allowed in downstream projects, so I propose we return to it. Further, I replaced an identity map array with a simple OrdinalMapper identity function. It might be confusing to use the OrdinalMapper, but in reading it's javadoc, I think it applies here.

The added test fails without this change.

[github-actions]

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[marianotepper]

@sam-herman please take a look at this PR. As it is an improvement over #536

[marianotepper]

LGTM

[marianotepper]

IMO, #555 is the preferred fix for this, so I think we should not merge this PR and go with the other one instead.