PHP 8.3–8.5 support, CI updates, Content-MD5 fix

[maksimovic]

Summary

• Add PHP 8.3, 8.4, and 8.5 to the CI test matrix
• Bump actions/checkout to v4 and codecov/codecov-action to v5
• Add replace block in composer.json for ralouphie/getallheaders
• Add composer test and composer test:coverage scripts
• Fix Content-Md5 → Content-MD5 header casing per IANA spec (Correct Content-MD5 header format ralouphie/getallheaders#12)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (141f16c) to head (31c96ea).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master        #4      +/-   ##
===========================================
+ Coverage   95.83%   100.00%   +4.16%     
===========================================
  Files           1         1              
  Lines          24        28       +4     
===========================================
+ Hits           23        28       +5     
+ Misses          1         0       -1     

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