Extend CI matrix to PHP 8.4 and 8.5

[morcen]

Problem

CI matrix coverage for PHP versions is inconsistent between the two test workflows, and neither covers PHP 8.5:

• .github/workflows/test-matrix.yml already tests php: [8.4, 8.3, 8.2] — 8.4 has been present since the v3.0.0 release, predating this issue.
• .github/workflows/run-tests.yml still only tests php: [8.3, 8.2] — no PHP 8.4 coverage at all.
• Neither workflow includes PHP 8.5, which has been released with zero CI signal in this repo.

Update (2026-08-05): the original framing ("CI matrix tests PHP 8.2 and 8.3 only") was already stale for test-matrix.yml as of this issue's filing — see also #133, which separately flags that test-matrix.yml's broader PHP coverage lives in an easily-overlooked, duplicate workflow from run-tests.yml. run-tests.yml, however, genuinely remains stuck on 8.2/8.3 with no merged change since.

Location

• .github/workflows/run-tests.yml
• .github/workflows/test-matrix.yml

Why it matters

Users installing on PHP 8.4 get real CI signal only from test-matrix.yml, and no signal at all on 8.5 from either workflow. Deprecations (e.g. implicit nullable parameters in 8.4) surface first as user bug reports rather than in CI.

Fix

1. Add PHP 8.4 to run-tests.yml's matrix so both workflows agree on supported versions.
2. Add PHP 8.5 to both run-tests.yml and test-matrix.yml.
3. Decide the support floor for the next tag.

(Note: consolidating these two workflows into one, as proposed in #133, would also resolve the "which workflow is authoritative" confusion this issue partly stems from.)

Sizing

Priority 20 (Impact 2, Risk 2, Effort 1). Estimated 1 hour plus fixing whatever breaks.

From the tech debt audit dated 2026-07-05.

[morcen]

Automated sync: checked current state — test-matrix.yml already includes PHP 8.4 (has since the v3.0.0 release, before this issue was filed), so the original "tests PHP 8.2/8.3 only" description was inaccurate for that file. run-tests.yml is still missing 8.4 entirely, and neither workflow has 8.5. Updated the issue body to reflect this and narrowed the ask to: bring run-tests.yml up to 8.4, and add 8.5 to both workflows.

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