Add unit tests for HtmlResponseFormatter

[WarLikeLaux]

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

What does this PR do?

Add unit tests for HtmlResponseFormatter: charset appending logic, content type preservation, data-to-content copy, and null data handling.

Before / After

File	Tests	Methods	Lines	MSI
HtmlResponseFormatter.php	0 → 9	0/1 → 1/1	0/5 (0%) → 5/5 (100%)	— → 100% (7/7)

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.56%. Comparing base (379b296) to head (c38519a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20778   +/-   ##
=========================================
  Coverage     78.56%   78.56%           
  Complexity    11568    11568           
=========================================
  Files           376      376           
  Lines         30750    30750           
=========================================
  Hits          24160    24160           
  Misses         6590     6590           

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