Random Redirect: test existing function compatibility

[chrisbliss18]

Proposed changes

• Add regression coverage for the Random Redirect function conflict fixed by Random Redirect: only declare the redirect function when nothing else has #51037.
• Run the compatibility check in a separate PHP process so a theme or plugin implementation can declare and hook jetpack_matt_random_redirect() before Jetpack loads its module.
• Require Jetpack's module twice to verify repeated loads remain idempotent: the existing implementation is preserved and the callback is not registered again.
• Expand behavior coverage for bare requests, invalid post-type fallback, POST requests, and unpublished posts.

This PR is stacked on #51037 so its test suite remains green. It can be retargeted to trunk after that PR merges.

Related product discussion/links

• Random Redirect: only declare the redirect function when nothing else has #51037

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. Start and install the Jetpack Docker environment if needed:

   jp docker up -d
   jp docker install

2. Run the Random Redirect tests:

   jp docker phpunit jetpack -- --filter=Random_Redirect_Test

3. Confirm all 12 tests pass, including test_loads_with_existing_random_redirect_function.

Red/green behavior was also verified directly with the standalone fixture:

• On the Jetpack 16.1 beta backport commit (5d8235ab7c), the fixture exits with status 255 and the original Cannot redeclare jetpack_matt_random_redirect() fatal.
• With Random Redirect: only declare the redirect function when nothing else has #51037 applied, the fixture prints OK, and the complete Random Redirect test class passes: 12 tests and 13 assertions.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the agent/random-redirect-function-conflict-test branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack agent/random-redirect-function-conflict-test

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/modules/theme-tools/random-redirect.php	39/51 (76.47%)	1.96%	-1 💚

Full summary · PHP report · JS report

[haqadn]

Looks good and tests well. Great job on the additional coverage.

The merge-base changed after approval.