Content Translation: Integrate Guidelines/Knowledge support

[wprashed]

Fixes #980

Description

Integrates WordPress Guidelines/Knowledge support into the Content_Translation Ability by implementing the guideline_categories() method declaring support for site, copy, and additional guidelines.

This allows site-wide guidelines, copy style rules, and translation glossaries to be passed to LLMs when requesting content translations.

Changes Included

• Implemented guideline_categories() in includes/Abilities/Content_Translation/Content_Translation.php returning array( 'site', 'copy', 'additional' ).
• Added unit test test_guideline_categories() in tests/Integration/Includes/Abilities/Content_TranslationTest.php.

Testing Instructions

1. Enable Guidelines / Knowledge and Content Translation experiments.
2. Verify wpai_content_translation Ability includes declared guideline categories (site, copy, additional) in its metadata.
3. Test executing Content Translation with guidelines enabled.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.38%. Comparing base (813c81d) to head (3df0aa5).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #987   +/-   ##
==========================================
  Coverage      75.37%   75.38%           
- Complexity      3357     3358    +1     
==========================================
  Files            138      138           
  Lines          12976    12978    +2     
==========================================
+ Hits            9781     9783    +2     
  Misses          3195     3195           

Flag	Coverage Δ
unit	75.38% <100.00%> (+<0.01%)	⬆️

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[dkotter]

Couple minor things that need updated. In addition, we have a standard PR template that we want each contributor to follow. It looks like the PR description here doesn't completely follow that (for instance, the Use of AI Tools is missing among a few other things)

[dkotter]

Please refer to our contributing guide. These should be set to x.x.x