Fix[Experiments]: Adopt character based count to gate features

[yogeshbhutkar]

What, Why, and How?

Closes #767

This PR standardizes AI feature content-length checks on character counts excluding spaces, instead of switching between words and characters based on locale.

This fixes cases where AI feature buttons stay disabled for sufficient CJK or similar non-space-delimited content when the site or user locale uses word-based counting. It also keeps minContentLength consistent by making the unit deterministic across users and languages.

The character thresholds are based on the previous word thresholds, using 5 characters as an approximate average word length. So a previous limit of 15 words now becomes 75 (15*5) characters. This keeps the limits calculated and proportional, rather than choosing new values arbitrarily.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5.5
Used for: Code reviews

Testing Instructions

1. Create a new post.
2. Confirm gated AI features show the minimum length message using character count.
3. Keep the site/editor locale set to English.
4. Add Japanese content to the post.
5. Confirm the gate is removed once the content reaches the required character length.
6. Use Content Resizing and confirm the changeset shows character counts correctly instead of “no change”.
7. Confirm the Shorten action works with sufficient Japanese content in an English locale.

Screenshots

Before	After

Changelog Entry

Changed - Use character-based count to gate AI Experiments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.51%. Comparing base (8da94a9) to head (cc285a8).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #802      +/-   ##
=============================================
+ Coverage      76.48%   76.51%   +0.03%     
  Complexity      1869     1869              
=============================================
  Files             89       89              
  Lines           7990     7996       +6     
=============================================
+ Hits            6111     6118       +7     
+ Misses          1879     1878       -1     

Flag	Coverage Δ
unit	76.51% <100.00%> (+0.03%)	⬆️

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Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[jeffpaul]

@hasanmehmood if you can connect your GitHub and WordPress.org accounts, then I can more formally credit you in the release post there and grant you the AI Contributor badge.