fix: resolve test hang with websockets 14+

[SamMorrowDrums]

Summary

Fixes #3708

The test suite hangs indefinitely when upgrading websockets from 13.x to 14.x or 15.x. This is because websockets 14.0 switched to a new asyncio implementation by default, which has different behavior around event loop handling.

Root Cause

The issue stems from how the TestServer manages event loops when running in a background thread:

1. In TestServer.serve(): The deprecated asyncio.get_event_loop() + loop.create_task() pattern doesn't work correctly with websockets 14+'s new asyncio handling
2. In serve_in_thread(): The server.run() method relies on asyncio.run() which doesn't play well with the threaded setup when websockets is involved

Changes

1. TestServer.serve(): Replace deprecated asyncio.get_event_loop() pattern with asyncio.create_task() which is the modern way to create tasks when already inside an async context.

2. serve_in_thread(): Instead of relying on server.run() which uses asyncio.run(), explicitly create a new event loop in the background thread with asyncio.new_event_loop() and asyncio.set_event_loop().

3. server fixture: Remove loop="asyncio" from Config() since we now manage the event loop manually, avoiding conflicts with websockets' asyncio implementation.

Testing

Tested locally with websockets 15.0.1 - all 43 integration tests pass without hanging.

[karpetrosyan]

Duplicate of #3693, but thank you very much for the contribution. I will mention you in the fix as a co author!