Fix ASGI streaming response start messages

[pr3etk]

Streaming Middleware Fix Report

Issue

SlowAPIASGIMiddleware forwarded the stored http.response.start ASGI message whenever it received an http.response.body message. Streaming responses emit multiple body messages, so the middleware sent the response-start message once per chunk. ASGI servers such as Uvicorn reject repeated response-start messages.

Changes

• Added a response_started flag to _ASGIMiddlewareResponder.
• Forwarded the response-start message, including rate-limit headers and error status handling, only on the first body message.
• Continued forwarding every body message unchanged.
• Added a regression test using FastAPI and StreamingResponse with three chunks.

Verification

• Before the fix, the regression test failed with assert 4 == 1 because four response-start messages were emitted.
• After the fix, pytest tests/test_streaming.py -q passed: 1 passed.
• The full suite after the fix reported 92 passed, 12 failed.
• All 12 failures were the same pre-existing Starlette compatibility failure: AttributeError: 'Starlette' object has no attribute 'route' in tests/test_starlette_extension.py.

Scope

No unrelated Starlette compatibility failures were changed. The fix is limited to ensuring that an ASGI response-start message is sent exactly once for streaming responses.

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