Build/Test Tools: Trim PHPUnit runs and add full-matrix labels in 7.0

[lancewillett]

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

Replaces #13541 with the explicit matrix lists and conditional exclusions from r63779, and adds the label-triggered full matrix from r63585 to the 7.0 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	66	54
Dispatch	66	146
PR with full-matrix label	66	146

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 #13637, #13639, and #13640 will need:

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

[desrosj]

Approving with the same call out as on #13637.

[lancewillett]

https://core.trac.wordpress.org/changeset/63892