fix: Prevent _prepare_request_call from mutating caller's headers dict

[vdusek]

Summary

_prepare_request_call in HttpClientBase was mutating the caller's headers dict in place — the if not headers: headers = {} guard only allocated a new dict when the caller passed None/empty, so a non-empty caller-owned dict was mutated directly. Users of the documented custom HttpClient / HttpClientAsync extension points that reuse a shared headers dict across call() invocations would see stale Content-Type / Content-Encoding headers leak into subsequent requests (e.g. a bodyless GET carrying Content-Encoding: gzip from a prior POST).

Fix: copy up-front with headers = dict(headers) if headers else {}. Added a regression test covering the aliased-dict case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (6c3b84d) to head (3e9bbae).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
- Coverage   95.36%   95.29%   -0.08%     
==========================================
  Files          45       45              
  Lines        5118     5117       -1     
==========================================
- Hits         4881     4876       -5     
- Misses        237      241       +4     

Flag	Coverage Δ
integration	95.29% <100.00%> (-0.08%)	⬇️

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