Commenting scaling feature

[karthick-murugan]

Closes #71668

This PR updates the useBlockComments hook to load comments in two stages:

Initial request: Fetch only top-level note comments.

Background request: Fetch all note comments (with intermediate results enabled).

Why?

The current commenting implementation may not scale well when there are many (hundreds or thousands) comments (or comment replies) on a post.

How

• Load only top-level comments first to render the sidebar immediately.
• Then fetch all remaining comments in the background using intermediate results.
• Enable the background fetch only after top-level data is ready, preventing sidebar flicker and scaling to large comment sets.

Testing Instructions

• Open a post with many comments (or create >100 notes).
• Open the collab sidebar.
• The sidebar loads quickly with top-level comments.
• Replies load gradually in the background.

Note: This is an initial workaround to improve loading behavior for large comment sets. The approach may evolve as we gather more community feedback, validate performance, and explore more robust long-term solutions.

Video

REC-20251127190803.mp4

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: karthick-murugan <karthickmurugan@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: TimothyBJacobs <timothyblynjacobs@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.