Optimize ParallelLeafReader to improve term vector fetching efficienc

[DivyanshIITB]

This PR optimizes ParallelLeafReader to avoid redundant term vector fetching.

• Replaces per-field term vector fetching with a single call per reader.
• Reduces complexity from O(n^2) to O(n).
• Improves performance when handling large numbers of fields.
• Verified via existing tests.

Closes #7926

[DivyanshIITB]

Just a gentle reminder
@vigyasharma

[vigyasharma]

Changes look good to me. Can you run ./gradlew tidy to fix formatting issues, and add a changes entry before we merge this?

[DivyanshIITB]

Changes look good to me. Can you run ./gradlew tidy to fix formatting issues, and add a changes entry before we merge this?

I successfully ran ./gradlew tidy and the built was successful.
Also added the changes entry in CHANGES.txt

[vigyasharma]

We generally keep a single bullet per changes entry. Details are already available in the pull request that the entry points too.

[DivyanshIITB]

I have modified CHANGES.txt as you said.

[vigyasharma]

I successfully ran ./gradlew tidy and the built was successful.

Github build is still failing on spotless (formatting). tidy will change and reformat offending files for you, you need to commit and push those changes.

[DivyanshIITB]

I successfully ran ./gradlew tidy and the built was successful.

Github build is still failing on spotless (formatting). tidy will change and reformat offending files for you, you need to commit and push those changes.

Thanks for the review @vigyasharma !
I have run ./gradlew tidy and pushed the formatting fixes. Let me know if there's anything else needed.

[vigyasharma]

Changes merged. Thanks @DivyanshIITB !