Add unit tests for ExitCode

[WarLikeLaux]

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

What does this PR do?

Add unit tests for ExitCode: getReason() for all 17 codes, unknown code fallback, late static binding, and constant values against FreeBSD sysexits(3) spec.

Before / After

File	Tests	Methods	Lines	MSI
ExitCode.php	0 → 24	0/1 → 1/1	0/1 (0%) → 1/1 (100%)	— → 100% (3/3)

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.58%. Comparing base (379b296) to head (e0cc526).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20777      +/-   ##
============================================
+ Coverage     78.56%   78.58%   +0.01%     
  Complexity    11568    11568              
============================================
  Files           376      376              
  Lines         30750    30750              
============================================
+ Hits          24160    24165       +5     
+ Misses         6590     6585       -5     

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