Implement online index backfill

[sudo-shashank]

Summary of changes

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes #7340

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• This pull request is based on an issue that a maintainer has accepted (see Before Opening a Pull Request).
• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit simplified code in branch shashank/index-backfill

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