Remove unused ai service class

[theaminulai]

What?

See #233

Removes the unused AI_Service layer (includes/Services/AI_Service.php), its test file, and the get_ai_service() helper.

Why?

AI_Service was introduced in #101 (0.2.1) as a proposed centralized layer for AI provider access, and #233 tracked refactoring the experiments to use it. A follow-up PR (#898) that wired several abilities through AI_Service was closed, since per [@jeffpaul's comment](#233 (comment)) and @dkotter's review on #898, the layer isn't used anywhere, hasn't been adopted by newer experiment PRs, and doesn't provide value over calling wp_ai_client_prompt() directly. Rather than carry the dead code forward, this PR removes it.

How?

• Deleted includes/Services/AI_Service.php (the create_textgen_prompt() wrapper and singleton).
• Deleted tests/Integration/Includes/Services/AI_ServiceTest.php.
• Removed the get_ai_service() helper and its use WordPress\AI\Services\AI_Service; import from includes/helpers.php. The ai_experiments_service_initialized action referenced only in the deleted test was never fired anywhere (do_action() for it doesn't exist in the codebase), so no hook needs replacing.
• Updated docs/ARCHITECTURE_OVERVIEW.md and docs/experiments/multi-provider-support.md to drop references to AI_Service (the includes/Services/ folder still exists for Guidelines.php).
• Added a changelog entry under Unreleased; left the historical 0.2.1 entry describing the original introduction untouched.
• Confirmed via a codebase-wide search that no ability, experiment, or feature calls AI_Service::get_instance() or get_ai_service() — all AI calls already go through wp_ai_client_prompt() directly, so this is a pure removal with no functional change.

Testing Instructions

1. Confirm nothing else references the removed class or helper:
   • Search the codebase for AI_Service and get_ai_service — no matches should remain outside historical CHANGELOG entries.
2. Run the PHPUnit integration suite to confirm no test depends on the removed class:
   • composer test
3. Spot-check that Excerpt Generation, Title Generation, Content Summarization, Alt Text Generation, and Image Generation still work end-to-end with a connected provider (they were unaffected by this change, but worth a smoke test).

[github-actions]

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Co-authored-by: theaminulai <theaminuldev@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.44%. Comparing base (7e92a70) to head (d1440a5).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #905   +/-   ##
==========================================
  Coverage      80.43%   80.44%           
- Complexity      2565     2566    +1     
==========================================
  Files            110      110           
  Lines          10448    10451    +3     
==========================================
+ Hits            8404     8407    +3     
  Misses          2044     2044           

Flag	Coverage Δ
unit	80.44% <100.00%> (+<0.01%)	⬆️

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[dkotter]

@theaminulai Thanks for the PR! In thinking about it a bit more, since the class is public and the helper function is also public, better to deprecate those and remove later than just remove entirely. Otherwise anyone that is relying on those will hit fatal errors.

I've pushed changes to handle that deprecation process but let me know if all of that looks good to you. In addition, would be great to get the PR title and description updated since we're not removing anything in this release, just deprecating