Split Available Experiments into Editor and Admin sections

[Ryujiyasu]

Summary

Closes #218

• Add a category field ('editor' or 'admin') to experiment metadata via the Experiment interface and Abstract_Experiment base class
• Split the single "Available Experiments" section into two distinct sections: Editor Experiments and Admin Experiments
• Reorder experiments to match the proposed structure in the issue
• Extract experiment item rendering into a reusable render_experiment_item() private method to avoid duplication

Screenshot

Screenshot will be added in a comment below.

Test plan

• Visit Settings > AI Experiments and verify experiments are grouped into "Editor Experiments" and "Admin Experiments" sections
• Verify Editor Experiments appear in order: Title Generation, Excerpt Generation, Content Summarization, Image Generation, Alt Text Generation
• Verify Admin Experiments shows: Abilities Explorer
• Toggle global experiments off and verify the disabled notice appears in the Editor section
• Enable/disable individual experiments and save — verify settings persist correctly
• Run composer lint and composer phpstan to verify coding standards

🤖 Generated with Claude Code

[github-actions]

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

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[codecov]

Codecov Report

❌ Patch coverage is 14.11765% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.45%. Comparing base (530ae7e) to head (12ded43).

Files with missing lines	Patch %	Lines
includes/Settings/Settings_Page.php	0.00%	71 Missing ⚠️
includes/Abstracts/Abstract_Experiment.php	33.33%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #263      +/-   ##
=============================================
- Coverage      55.91%   55.45%   -0.46%     
- Complexity       509      513       +4     
=============================================
  Files             32       32              
  Lines           2611     2647      +36     
=============================================
+ Hits            1460     1468       +8     
- Misses          1151     1179      +28     

Flag	Coverage Δ
unit	55.45% <14.11%> (-0.46%)	⬇️

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[Ryujiyasu]

SCREENSHOT is here

[dkotter]

@Ryujiyasu There's already a separate PR open to solve this (#232). Curious if there's anything unique in this PR we should be considering or if this should be closed as a duplicate?

[Ryujiyasu]

@dkotter Thanks for pointing that out! I wasn't aware of #232 when I opened this. This was a great learning experience for me, but since #232 already covers the same ground, I'll go ahead and close this as a duplicate. 🙏