Expose URL Metric to Chrome DevTools for agents

[faisalahammad]

Summary

Fixes #2553

Adds a small Optimization Detective extension module that registers with Chrome's new DevTools for agents third-party tools API. It exposes two read-only tools, get_od_url_metric and get_od_element_data, so an AI coding agent can inspect the URL Metric being collected for the current page load while debugging performance. Only active when WP_DEBUG is enabled.

Relevant technical choices

• Shipped as a new extension module (devtools-discovery.js), same mechanism Embed Optimizer and Image Prioritizer already use via the od_extension_module_urls filter. No changes to core detect.js.
• The module only calls the getRootData() / getElementData() functions that are already handed to every extension's initialize(), so there's no new state tracking.
• The filter is only added when WP_DEBUG is true, since this is a debugging feature and should cost nothing in production (no extra listener, no extra data collection).
• Added PHPUnit coverage for the new filter callback and the conditional hook registration, plus a short note in the extension docs.

How to test

1. Build the plugin and run a local WP site with WP_DEBUG set to true.
2. Configure the Chrome DevTools MCP server with --categoryExperimentalThirdParty=true.
3. Ask an agent to list third-party developer tools on a page. You should see an "Optimization Detective" group with get_od_url_metric and get_od_element_data.
4. Invoke get_od_url_metric, confirm it returns the current URL Metric (viewport + tracked elements).
5. Set WP_DEBUG to false, reload, confirm the tool group no longer appears.

Also ran composer test:optimization-detective, composer lint:optimization-detective, npm run phpstan, and npm run tsc locally, all passing.

[github-actions]

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Co-authored-by: faisalahammad <faisalahammad@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.37%. Comparing base (8d70987) to head (703d128).

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2571      +/-   ##
==========================================
+ Coverage   70.35%   70.37%   +0.02%     
==========================================
  Files          91       91              
  Lines        7867     7873       +6     
==========================================
+ Hits         5535     5541       +6     
  Misses       2332     2332              

Flag	Coverage Δ
multisite	70.37% <100.00%> (+0.02%)	⬆️
single	35.14% <0.00%> (-0.03%)	⬇️

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