Fix artisan serve crash: make uvicorn WebSocket backend configurable

[tmgbedu]

Problem

artisan serve crashed with ModuleNotFoundError: No module named 'websockets' on apps that never use WebSockets. The serve command hardcoded ws="websockets-sansio" in the uvicorn.run kwargs, and uvicorn imports that backend at startup — requiring the optional websockets package even when no WebSocket is ever opened.

Fix

• Replace the hardcoded ws="websockets-sansio" with a proper --ws option accepting the values uvicorn supports: auto, none, websockets, websockets-sansio, wsproto.
• Resolution order: CLI flag > fastapi.ws config > default.
• Safe default auto — uvicorn resolves it lazily and never requires the websockets package unless a WebSocket connection is actually opened, so serve always boots cleanly.
• Invalid --ws values print a clear error listing the allowed backends and exit non-zero, instead of raising a stack trace.
• Added a ws field to FastAPIConfig (env APP_WS, default auto) so the backend is configurable and documented.
• host / port / reload behaviour is unchanged.

Verification

• Reproduced the crash in an isolated env with neither websockets nor wsproto installed: ws="auto" and ws="none" load cleanly (ws_protocol_class=None), while ws="websockets-sansio" (the old hardcoded value) raises ModuleNotFoundError — confirming the root cause and the fix.
• WebSocket backends still work when the user opts into a backend whose package is installed.

Tests

• Updated the default-backend assertion to expect auto.
• Added TestWsOption covering: flag pass-through, websockets-sansio opt-in, none, config-driven selection, CLI-overrides-config, and invalid value (non-zero exit, uvicorn.run not called, error lists allowed values).
• Full suite: 163 passed (tests/fastapi/ + config tests).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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