RTC: Fix collaborator cursor rendering in table cells

[karthikeya-io]

What?

Closes #76812

This PR fixes an issue where collaborator cursors fail to render in Table block cells during a Real-Time Collaboration (RTC) session.

Why?

• Depth Failure: To get the block Y.Map the code checked 2 levels: Y.Text -> attributes Y.Map -> block Y.Map. While this works for other blocks, Table text is buried much deeper (inside Rows and Cells). The original logic would stop too early, fail to find the block ID, and return null.
• No Cell Identity: The individual Table cells didn't have a unique ID in the HTML (No identifier). This meant the cursor overlay had no way to know which cell a user was typing in.
• Offset Errors: Searching the entire table instead of a specific cell caused character counts to shift, making cursors appear in the wrong position or vanish

How?

Updated the selection flow to handle deep nesting:

• In post-editor-awareness.ts replaced the fixed 2-level check with a while loop. This loop walks up the data tree until it finds the block ID.
• Added a unique identifier to each Table cell's RichText component. This adds a data-wp-block-attribute-key to the HTML so the cell can be identified.
• Added function getYjsValueByPath to get the Y.Text object for that specific cell using the attributetKey like body[0].cells[1].content.
• Updated the rendering logic in cursor-dom-utils.ts to use the attributeKey. Instead of searching the whole table, it now jumps directly to the specific cell element before counting characters.
• Updated the ResolvedSelection type and the selection resolver to include the attributeKey, ensuring the cell's address is passed correctly.

Testing Instructions

1. Open a post in two different browser windows.
2. Add a Table block and create a few rows and columns.
3. In Window A, click into any cell and start typing.
4. Verify: In Window B, the colored cursor for User A should appear exactly in the correct cell.
5. Move to different cells and verify the cursor follows accurately to each one.

Screenshots or screencast

Before	After
Screen.Recording.2026-04-15.at.7.18.34.PM.mov	Screen.Recording.2026-04-15.at.7.08.32.PM.mov

Use of AI Tools

Drafted with Gemini assistance and reviewed it before submission

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Co-authored-by: karthikeya-io <karthikeya01@git.wordpress.org>
Co-authored-by: alecgeatches <alecgeatches@git.wordpress.org>

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