fix: Keep HttpResponse isinstance checks from consuming streamed responses

[vdusek]

HttpResponse now comes from typing_extensions, whose runtime protocol check looks attributes up statically. The typing implementation on Python 3.11 calls hasattr, which evaluates properties, so isinstance(response, HttpResponse) on an unread streaming response either raised or silently buffered the whole body.

Adds streaming regression tests (unit and integration) and a parametrized built-in-client fixture they run on.

Split out of #1006. Stacked on #1009.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (e38c0b5) to head (df8a08d).

Additional details and impacted files

@@                    Coverage Diff                     @@
##           fix-try-import-masking    #1010      +/-   ##
==========================================================
- Coverage                   94.89%   94.87%   -0.02%     
==========================================================
  Files                          58       58              
  Lines                        5366     5367       +1     
==========================================================
  Hits                         5092     5092              
- Misses                        274      275       +1     

Flag	Coverage Δ
integration	92.23% <100.00%> (-0.08%)	⬇️
unit	85.14% <100.00%> (+0.48%)	⬆️

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