Run tests on PHP 8.4 and update test suite

[clue]

Builds on top of #36 and clue/framework-x#267
Supersedes / closes #39

[Copilot]

Pull Request Overview

Adds PHP 8.4 to the CI test matrix and updates CI environment/platform configuration while adjusting PHPUnit legacy configuration to enable full error reporting.

• Added PHP 8.4 to matrix and updated actions/checkout version.
• Updated OS runner labels to newer images.
• Added error_reporting=-1 to legacy phpunit config.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
phpunit.xml.legacy	Enable full error reporting during tests.
.github/workflows/ci.yml	Updated runner OS versions, added PHP 8.4, and bumped checkout action version.

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