Fix meta description UTF-8 character count

[yusufhay]

What?

Updates meta description generation to pass UTF-8 explicitly when calculating the generated description character count.

Why?

The generated meta description response includes a character_count value. Other multibyte length checks in the plugin already pass UTF-8 explicitly, and this keeps meta description output consistent for multilingual content.

How?

Changes the mb_strlen() call used for the generated meta description output from relying on PHP internal encoding to explicitly using UTF-8.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex / ChatGPT
Model(s): GPT-5
Used for: Scanning the codebase for small correctness issues and drafting the minimal change. I reviewed the change and PR description before submitting.

Testing Instructions

1. Install and activate this PR build of the AI plugin.
2. Connect a provider that supports text generation.
3. Enable the Meta Description experiment.
4. Open or create a post with non-English / multibyte content.
5. Generate a meta description.
6. Confirm the generated result still appears normally and the returned character_count reflects the generated UTF-8 text.

Screenshots or screencast

Not applicable. This PR does not change the UI.

Changelog Entry

Fixed - Use explicit UTF-8 encoding for generated meta description character counts.

[github-actions]

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Co-authored-by: yusufhay <yusufmudagal@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.68%. Comparing base (64e8032) to head (24347af).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...es/Abilities/Meta_Description/Meta_Description.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #655      +/-   ##
=============================================
+ Coverage      74.57%   74.68%   +0.10%     
  Complexity      1754     1754              
=============================================
  Files             85       85              
  Lines           7548     7549       +1     
=============================================
+ Hits            5629     5638       +9     
+ Misses          1919     1911       -8     

Flag	Coverage Δ
unit	74.68% <0.00%> (+0.10%)	⬆️

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