Add unit test for PHPUnit\Metadata\Parser\Registry singleton

[miladev95]

Adds a focused unit test that verifies the singleton behavior of PHPUnit\Metadata\Parser\Registry::parser() and that the registry can be recreated after resetting its private static instance.

testParserReturnsParserInstanceAndIsSingleton — asserts Registry::parser() returns an instance implementing PHPUnit\Metadata\Parser\Parser and that repeated calls return the same instance (singleton).

testParserRecreatedAfterResettingInstanceWithReflection — uses reflection to reset the private static instance, asserts a new Parser is created and is not the same object as before (tests recreation path).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (61804e3) to head (448344a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6431   +/-   ##
=========================================
  Coverage     95.85%   95.85%           
  Complexity     7281     7281           
=========================================
  Files           789      789           
  Lines         22404    22404           
=========================================
  Hits          21476    21476           
  Misses          928      928           

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[sebastianbergmann]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided not to merge it as the method in question is already sufficiently tested.