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 (f57e9e7) to head (08553b3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #1010 +/- ##
=======================================
Coverage 94.87% 94.87% =======================================
Files 58 58 Lines 5366 5367 +1 =======================================
+ Hits 5091 5092 +1 
Misses 275 275 

Flag	Coverage Δ
integration	92.09% <100.00%> (-0.13%)	⬇️
unit	85.14% <100.00%> (+0.48%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.