Optimizes live docs computes for force merge in NativeEngines990KNNVectorWriter

[shatejas]

Description

Currently live docs are computed with linear complexity even when it does not have deleted docs. This PR skips computing it

Testing

• UTs and javadocs in progression. Need initial feed back

Related Issues

https://github.com/opensearch-project/k-NN/issues/2134

Check List

• [x] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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[navneet1v]

Please also update the changelog.