Add unit tests for PhpMessageSource

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues

What does this PR do?

Add unit tests for PhpMessageSource: getMessageFilePath with fileMap, fallback chain (en-US → en), and loadFallbackMessages merge logic.

Before / After

File	Tests	Methods	Lines	MSI
PhpMessageSource.php	0 → 26	0/4 → 4/4	0/39 (0%) → 39/39 (100%)	— → 91% (67/73)

Escaped mutants (all equivalent)

PhpMessageSource.php (6): ProtectedVisibility ×3 - L103, L135, L157, always equivalent. CastString ×2 - L76, L137, defensive cast on already-typed parameter. CastArray ×1 - L125, caller loadMessages applies (array) on the return value.

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.57%. Comparing base (379b296) to head (8f76f1a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20776   +/-   ##
=========================================
  Coverage     78.56%   78.57%           
  Complexity    11568    11568           
=========================================
  Files           376      376           
  Lines         30750    30750           
=========================================
+ Hits          24160    24162    +2     
+ Misses         6590     6588    -2     

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