Fix the runtime matrix's nginx cells; run an nginx cell on PRs

[dknauss]

Summary

What changed?

Since #106, Runtime matrix smoke on main has failed on every push: runs #525, #529, and #531. In each run the same three cells fail, all nginx (nginx-php81-wp67, nginx-php82-wplatest, nginx-php83-wplatest), and all seven Apache cells pass. Every other recent workflow is green: CI, CodeQL, Release, the WordPress.org deploy, Dependency audit, Node compatibility, and Demo Link Monitor.

Cause. The settings PATCH that #106 added to the smoke test gets nginx's own 405 Not Allowed, so the request never reaches PHP. The smoke test sent REST requests to /?rest_route=…. For a directory request like /, nginx's index module only hands GET, HEAD, and POST to index.php. PATCH, PUT, and DELETE fall through to the static-file handler, which answers with 405. #105's write steps used only GET and POST, which is why its full-matrix run passed.

Sites are not affected. With plain permalinks WordPress builds REST URLs as index.php?rest_route=…, and pretty permalinks (/wp-json/…) reach index.php through try_files.

Fix:

• scripts/runtime-matrix/smoke.sh sends every REST request to index.php?rest_route=… through a single REST_URL, the form WordPress generates itself.
• Why it wasn't caught before merge: the pull-request matrix had only Apache cells. It now also runs nginx-php83-wplatest, so this PR's own CI tests the fix on nginx.
• The README lists the four PR cells, and the CHANGELOG has a line for the fix.

Validation

• npm run lint:js: no JS changes
• npm run lint:css: no CSS changes
• npm run lint:i18n: no source strings changed
• npm run test: no plugin code changes
• npm run build: no plugin code changes
• bash -n on smoke.sh, and the workflow YAML parses; the PR matrix has 4 cells, including nginx.
• Reproduced nginx's behavior with nginx:1.27-alpine. A full PHP-FPM reproduction was blocked by Docker Hub rate limits, so this PR's new nginx cell is the end-to-end proof.
• Tested in the block editor: test infrastructure only.

Checklist

• Linked issue or explained why none was needed: the failing main runs are linked above.
• No sensitive details disclosed publicly

🤖 Generated with Claude Code

https://claude.ai/code/session_01KbVuMPN7YTcV27vwsG3LZ2

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.50%. Comparing base (6a3d290) to head (dc1517e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #109   +/-   ##
=========================================
  Coverage     87.50%   87.50%           
  Complexity       22       22           
=========================================
  Files            55       55           
  Lines          6089     6089           
  Branches        597      597           
=========================================
  Hits           5328     5328           
  Misses          280      280           
  Partials        481      481           

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