Upgrade PHPUnit to 10 #622

Summary

Move the test framework from PHPUnit 9 to PHPUnit 10 (phpunit/phpunit from ^9.0 to ^10.5).

Why

PHPUnit 9 is no longer the current line. The PHP 8.1 minimum (#595) is what makes PHPUnit 10 possible, since it requires PHP 8.1 or newer.

Scope

• Raise phpunit/phpunit to ^10.5 in composer.json. Composer resolves it together with the newer sebastian/* packages and phpunit/php-code-coverage 10. mockery/mockery and mikey179/vfsstream are already at their latest versions.
• Migrate phpunit.xml to the PHPUnit 10 schema: verbose and convertDeprecationsToExceptions are gone, and <coverage processUncoveredFiles> becomes <source>.
• Check that the composer test flags and the CI coverage upload still work.
• Fix whatever the suite reports under PHPUnit 10, such as risky tests, output during tests, and global-state handling in the @runInSeparateProcess and @preserveGlobalState tests. The @dataProvider annotations should keep working; this needs confirming.
• The checkpoints are written after a first run on PHPUnit 10 shows the real failures.

Acceptance Criteria

• composer.json requires phpunit/phpunit ^10.5
• phpunit.xml uses the PHPUnit 10 schema without deprecation warnings
• the suite passes on PHP 8.1, 8.2 and 8.3, and CI is green, with coverage still produced
• no test is skipped or removed only to make the upgrade pass