test: reproduce baseline malformed WebSocket upgrade returning 501 #144

Problem

On clean main 5d855223e3ec8797b3c4ad7d01644a8a5145b501, the server WebSocket test expects HTTP 400 for an upgrade without a key but receives 501. Reproduced on Python 3.12.13 and 3.13.13/Linux, also after removing HTTP(S)_PROXY variables. The AK/SK feature branch reproduces the same result without changing server code.

Reproduction

Install server-sdk[dev] and run:

cd server-sdk
pytest tests/test_server/test_app_agents_transport.py::TestResponsesWebSocket::test_upgrade_without_a_key_is_a_400_not_a_hang -q

Full Python 3.12 server suite: 874 pass, 1 fail, 1 skip. No cloud credentials required.

Acceptance

Diagnose the 501 response and make the supported Python matrix and local CI pass while retaining malformed-upgrade coverage. This baseline blocks declaring local make ci green for #143 and is outside client authentication scope.

Supported CI comparison

The managed-key candidate #145 passed the full supported remote CI matrix. These failures remain specific to the local workspace reproduction; their cause is not claimed to be fixed. Passing required remote checks can satisfy the feature merge gate while this environment follow-up stays open.