Build/Test Tools: Trim PHPUnit runs and add full-matrix labels in 7.1

[lancewillett]

Trac ticket: https://core.trac.wordpress.org/ticket/66069

Replaces #13540 with the explicit matrix lists and conditional exclusions from r63779, and adds the label-triggered full matrix from r63585 to the 7.1 branch. The database trim follows r63584.

Routine pushes and pull requests keep the branch's current boundary PHP versions and trim the database list. Adding Full PHPUnit Matrix starts a separate full run without a code push; subsequent commits keep full coverage while the label remains. Dispatched runs also test the full PHP and database lists, so scheduled testing of older branches retains that coverage.

MySQL and MariaDB jobs	Before	After
Routine PR	72	54
Dispatch	72	160
PR with full-matrix label	72	160

Full runs restore the historical PHP list: 7.4, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5. Routine runs retain 7.4, 8.0, 8.5. Existing memcached, alternate-domain, report, HTML API, and fork configurations are preserved. The counts above exclude innovation-release and other job groups.

The separate label workflow follows trunk's implementation and matches this branch's existing PHPUnit path filters. Unrelated labels do not enter the reusable workflow or cancel active tests. Contributors without label access can ask a reviewer to add it, as documented in tests/phpunit/README.md.

Validation

• Actionlint and git diff --check passed.
• 54 exact matrix comparisons for this branch covered push, PR, schedule, dispatch, and called-workflow contexts, with and without the full-matrix label. Full lists were checked against the pre-trim workflow history; routine jobs were checked against the current branch with only the intended database removals.
• Verified unchanged special jobs and reusable-workflow inputs, matching normal/label path filters, and the existing concurrency behavior.
• Zizmor's existing branch findings are unchanged. The new local reusable-workflow reference adds one low-severity self-repository syntax suggestion, matching trunk's existing pattern.
• GitHub Actions results will be available on this PR. Live label-triggered execution has not yet been verified on this branch.

This needs a second committer's review before SVN landing. Keep #66069 open until all four branch changes have landed.

Use of AI tools

AI assistance: Yes.
Tool: Codex.
Model: GPT-6.
Used for: Workflow implementation, matrix validation, review, and this description. Reviewed by me.

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[lancewillett]

This PR, along with #13638, #13639, and #13640 will need:

1. at least 1 approval
2. at least 2 core committer as 2nd reviewer

[desrosj]

Reiterating what I stated in Slack:

I don't really love how we have guarded running the full matrix behind having the necessary permissions to add a label on the repository (which is currently only Core Committers, the Security Team, and less than a dozen other contributors). I get the intention, but it feels like we are needlessly blocking good contributors from being able to run the full test suite to validate their PRs.

I'm not convinced that additional contributor friction and the bottle neck of essentially needing a Core Committer to add a label here is the right way to limit the number of jobs run. I also don't love that we need an entirely new workflow just to make this happen. I read through the discussion and reasoning on #13448 so I understand why it's functionally required. But it doesn't feel like the best solution to the problem at the moment.

This PR matches r63585 from trunk, so approving in that sense and willing to give this a shot.

[lancewillett]

https://core.trac.wordpress.org/changeset/63891