test: e2e HTTP against granian --interface rsgi #12

Context

• tests/test_asgi.py uses the ASGI transport; the RSGI path is not exercised end-to-end with a real HTTP server.

Work

• In CI, subprocess to start granian on 127.0.0.1:<port> with a minimal app module, httpx GET /, assert 200, then terminate. Use a sync httpx client with retry on port bind races.

Flakiness

• Port allocation: use a random high port, retries, or pytest fixtures; document Windows caveats if the job is Linux-only at first.

Acceptance criteria

• Green on Linux at minimum; mark optional for Windows if flaky.