Honor streaming responses in TestClient

[ShipItAndPray]

Summary

• keep the test client portal open while the app task streams the response body
• return a real sync byte stream after the first http.response.body instead of buffering the full body up front
• add a regression test that proves client.stream(...) becomes available before the streaming response completes

Testing

• uv run pytest tests/test_testclient.py -k stream_response_is_available_after_first_chunk -q
• uv run pytest tests/test_testclient.py -q
• uv run pytest tests/test_templates.py -q
• uv run ruff check starlette/testclient.py tests/test_testclient.py

Fixes #1102