Do not hang forever if test server fails to start

[akx]

Summary

I was wondering why the test suite just hung forever, and finally with pytest tests/ -vvvx --log-cli-level=DEBUG -s I found that:

[Errno 48] error while attempting to bind on address ('127.0.0.1', 8000): [errno 48] address already in use

– yep, I was working on something else on port 8000 – and so, server.running never becomes True, and tests just hang.

This PR:

• Makes sure errors from server.run() (in the secondary thread) are caught and propagated immediately to the controlling loop
• Adds timeouts, so even if something even more unexpected happens, the suite will error out instead of hanging forever.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  • No separate tests for testing the test fixture 😄
• I've updated the documentation accordingly.
  • No user-facing or developer documentation changes needed. (Noting that port 8000 must not be in use is a separate thing to maybe document, I think...)

[lovelydinosaur]

@akx Thanks... I've shifted my development efforts over to this comprehensive redesign, due to it's significantly reduced complexity.

See also HTTPX 1.0 design proposal docs in progress.

---

For comparison...

$ pip install httpx --pre
Downloading httpx-1.0.dev3-py3-none-any.whl (34 kB)
Downloading certifi-2026.1.4-py3-none-any.whl (152 kB)
Installing collected packages: certifi, httpx

$ pip install httpx
Downloading httpx-0.28.1-py3-none-any.whl (73 kB)
Downloading httpcore-1.0.9-py3-none-any.whl (78 kB)
Downloading anyio-4.12.1-py3-none-any.whl (113 kB)
Downloading idna-3.11-py3-none-any.whl (71 kB)
Downloading certifi-2026.1.4-py3-none-any.whl (152 kB)
Downloading h11-0.16.0-py3-none-any.whl (37 kB)
Installing collected packages: idna, h11, certifi, httpcore, anyio, httpx

$ pip install requests
Downloading requests-2.32.5-py3-none-any.whl (64 kB)
Downloading certifi-2026.1.4-py3-none-any.whl (152 kB)
Downloading charset_normalizer-3.4.4-cp313-cp313-macosx_10_13_universal2.whl (208 kB)
Downloading idna-3.11-py3-none-any.whl (71 kB)
Downloading urllib3-2.6.3-py3-none-any.whl (131 kB)
Installing collected packages: urllib3, idna, charset_normalizer, certifi, requests