Feature: Disable Review notes until minimum content threshold is reached

[coderGtm]

What?

Closes #390

Adds the same minimum-content gating used by Content Summarization to Review Notes, requiring at least 100 characters before Review Notes can run, making that threshold filterable, and showing descriptive disabled-state text in the editor UI.

Why?

Review Notes could previously be triggered on very short or empty post content, which does not provide enough content for meaningful AI review suggestions.

Content Summarization already avoids this by requiring a minimum amount of content before enabling the feature. This change aligns Review Notes with that existing behavior so users get a more consistent UX across AI editor features, and so Review Notes only becomes available when there is enough content to review.

How?

This PR updates the Review Notes experiment to follow the same enablement pattern as Content Summarization.

Implementation details:

• Adds a new filter, wpai_review_notes_min_content_length, with a default value of 100
• Localizes the minimum content length to the editor script as minContentLength
• Adds shared Review Notes availability logic in the editor based on character count
• Disables the main Generate Review Notes button when post content is below the threshold
• Shows descriptive helper text explaining when Review Notes becomes available
• Disables the single-block Generate Review Note menu action when the post content is below the threshold
• Prevents short-content posts from triggering Review Notes requests
• Adds integration test coverage for the default and filtered minimum content length
• Adds e2e coverage for the disabled/enabled Review Notes states and the single-block disabled state
• Updates the empty-post e2e scenario to reflect the new minimum-length gating behavior

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): GPT-5-class model via Copilot
Used for: Implementation guidance, test updates, and PR drafting; final code and description were reviewed and edited by me.

Testing Instructions

1. Enable Experiments globally in the AI plugin settings.
2. Enable the Review Notes experiment.
3. Open a new post in the editor with content shorter than 100 characters.
4. Open the post settings sidebar.
5. Confirm the Generate Review Notes button is visible but disabled.
6. Confirm the helper text says Review Notes will be available at 100 characters.
7. Add enough content so the post reaches at least 100 characters.
8. Confirm the Generate Review Notes button becomes enabled.
9. Select a reviewable block and open the block options menu.
10. Confirm Generate Review Note is disabled when the post is below the threshold.
11. Confirm Generate Review Note becomes available once the post content meets the threshold.
12. Optionally add a filter for wpai_review_notes_min_content_length and verify the threshold changes accordingly.

Screenshots or screencast

Before	After
Review Notes could be triggered on empty or very short content.	Review Notes is disabled until the post reaches the minimum content length, with descriptive helper text shown.

Changelog Entry

Changed - Require a minimum content length before enabling Review Notes, with a filterable threshold and descriptive editor messaging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.35%. Comparing base (94f5d40) to head (35adacb).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #545      +/-   ##
=============================================
+ Coverage      71.20%   71.35%   +0.15%     
  Complexity      1150     1150              
=============================================
  Files             67       67              
  Lines           5563     5565       +2     
=============================================
+ Hits            3961     3971      +10     
+ Misses          1602     1594       -8     

Flag	Coverage Δ
unit	71.35% <100.00%> (+0.15%)	⬆️

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[github-actions]

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Co-authored-by: coderGtm <gautam23@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: vishnupprajapat <vishnuprajapat@git.wordpress.org>
Co-authored-by: pcprajapatcms <phoolchandcms@git.wordpress.org>

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[jeffpaul]

@coderGtm you'll likely want to either branch off #528 or wait for that to get merged in as that changes naming for the feature a good bit and will cause conflicts here