fix: decode HTTP headers as latin1 instead of utf8

[mcollina]

Summary

• Fixes the new interceptor API (onResponseStart) to decode HTTP headers as latin1 instead of UTF-8
• Per HTTP spec, header values are byte strings that should be decoded using isomorphic decode (latin1)
• Removes the content-disposition workaround that was needed when headers were inconsistently decoded (see http: unify header treatment node#46528)

Test plan

• Added unit tests for latin1 decoding in parseHeaders and parseRawHeaders
• Verified existing "Receiving non-Latin1 headers" test still passes
• All unit tests pass (npm run test:unit)
• Linting passes

Fixes #4753

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (e2aeb52) to head (143550b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4768      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files         109      109              
  Lines       34029    34007      -22     
==========================================
- Hits        31738    31715      -23     
- Misses       2291     2292       +1     

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