Testing: Add PHP code coverage collection for e2e tests

[priethor]

What

Adds PHP code coverage collection during e2e test runs, with Codecov integration.

Why

Currently, only JavaScript coverage is collected during e2e tests. This adds visibility into which PHP code paths are exercised by the e2e test suite, helping identify untested server-side code.

How

1. Adding an mu-plugin that uses Xdebug's coverage mode to collect PHP coverage data.
2. When Playwright tests run with PHP_COVERAGE_ENABLED=true, they send an X-PHP-Coverage header that triggers the mu-plugin to record coverage for each request.
3. Since PHP processes are stateless and each request is a new process, coverage is written to individual JSON files, which are then merged into LCOV format by a Node.js script
4. Finally, the merged report is uploaded to Codecov.

Testing Instructions

If everything goes well, the Codecov comment in this PR should show a significant increase in PHP coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.46%. Comparing base (1e1f998) to head (4636f28).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@              Coverage Diff              @@
##              trunk     #343       +/-   ##
=============================================
+ Coverage     34.07%   62.46%   +28.38%     
  Complexity     4410     4410               
=============================================
  Files           291      296        +5     
  Lines         36914    26261    -10653     
=============================================
+ Hits          12578    16403     +3825     
+ Misses        24336     9858    -14478     

Flag	Coverage Δ
e2e	?
e2e-js	44.17% <ø> (?)
e2e-php	99.85% <ø> (?)
javascript	15.07% <ø> (ø)
phpunit	29.97% <ø> (ø)

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[github-actions]

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