refactor: remove unused CacheHint.to_http_header

[nogueira-raphael]

Summary

• Remove CacheHint.to_http_header() method that was not used in production code
• Remove corresponding unit tests and unused pytest import
• HTTP header generation is only needed at the response level via ResponseCachePolicy.to_http_header(), not on individual field hints

Test plan

• All 139 tests passing
• mypy clean
• Verified no production code references CacheHint.to_http_header()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.41%. Comparing base (34643bc) to head (a39eca0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   71.55%   71.41%   -0.14%     
==========================================
  Files          34       34              
  Lines        1030     1025       -5     
  Branches      163      162       -1     
==========================================
- Hits          737      732       -5     
  Misses        254      254              
  Partials       39       39              

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