Enhance Content Summary block detection to nested blocks

[Intenzi]

What?

Closes #809

Enhances the Content Summarization experiment to detect Content Summary block inside nested-level blocks.

Why?

It solves the problem of new Content Summary block insertion when we want to regenerate the content summary if existing one is moved inside a nested block level such as inside a Group/Column block.
This PR ensures that the existing Content Summary block is found and edited instead.

How?

The fix is performed by implementing a recursive function named findSummaryBlock.

• Recursively performs Depth First Search to identify the Content Summary block's existence.

This function replaces the existing search logic for the Content Summary block.

• Implemented E2E test validating the same.

Additionally fixed a performance issue of the useSelect hook re-evaluating and running on every single render due to missing dependancy array. This was fixed by adding [] as empty dependancy array.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Gemini 3.5 Flash (Low)
Used for: Identifying relevant code file, deciding on implementation for enhancement. E2E test was implemented via it. The performance fix was detected by the model. Final implementation and tests were reviewed and edited by me.

Testing Instructions

• Enable Content Summarization Experiment.
• Create a new post with minimum required content for the button to enable.
• Click the Generate Summary button present inside the Post sidebar.
• Create a Group/Column block and place the generated Content Summary group block inside it.
• Identify that the button still states Regenerate Summary - (inside both Post sidebar and selected Block hovering menu bar)
• Click Regenerate Summary.
• Identify that the existing Content Summary group block has been modified instead of inserting a new block.

Screenshots or screencast

Visual Cues:

Manual Testing Video (from the step of Regenerate Summary click):

Enhancement.Complete.-.Content.Summarization.mov

Changelog Entry

Changed - Content Summary block detection from top-level only to checking nested blocks as well.

[github-actions]

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Co-authored-by: Intenzi <intenzi@git.wordpress.org>
Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.50%. Comparing base (30fc728) to head (3309fa2).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #810   +/-   ##
==========================================
  Coverage      75.50%   75.50%           
  Complexity      2086     2086           
==========================================
  Files             99       99           
  Lines           8626     8626           
==========================================
  Hits            6513     6513           
  Misses          2113     2113           

Flag	Coverage Δ
unit	75.50% <ø> (ø)

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[Intenzi]

Raised #811 to increase gh action run timeout which was causing the E2E test job to cancel mid-run.