[Fix]: Issue 264 | Anthropic with Title Gen

[TylerB24890]

What?

Closes #264

This PR removes the multiple candidates from the Title Generation experiment and makes it return only one. All E2E and unit tests updated accordingly.

Why?

As described in #264 -- Anthropic and OpenAI no longer support multiple candidate responses. This PR ensures the title generation logic continues to work even if only one of those models is currently connected.

How?

Remove all references to candidates within the Title Generation experiment. With the removal of multiple candidates per response, the modal UI has been removed and titles are just inserted into the post directly. Editors can then regenerate the title as needed using the existing toolbar button.

Use of AI Tools

• AI Assistance: Yes
• Tool(s): Claude Code (Opus 4.6)
• Used for:
  • Outlining everywhere candidates were referenced and used
  • Update E2E and Unit Tests accordingly

Testing Instructions

1. Ensure the "Title Generation" experiment is enabled.
2. Ensure only an Anthropic connector is active.
3. Navigate to a post and click the "Regenerate" button.
4. Confirm a title is generated and inserted into the post title field.

[github-actions]

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Co-authored-by: TylerB24890 <tyb@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 43.75000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.04%. Comparing base (ab57fa9) to head (5290d7c).
⚠️ Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
...es/Abilities/Title_Generation/Title_Generation.php	43.75%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #341      +/-   ##
=============================================
+ Coverage      57.95%   58.04%   +0.08%     
- Complexity       629      630       +1     
=============================================
  Files             46       46              
  Lines           3204     3189      -15     
=============================================
- Hits            1857     1851       -6     
+ Misses          1347     1338       -9     

Flag	Coverage Δ
unit	58.04% <43.75%> (+0.08%)	⬆️

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