Use asyncio.run in StatelessServer.run

[puneetdixit200]

StatelessServer.run() currently calls asyncio.get_event_loop() before running arun(). On Python 3.14, a fresh main thread may not have a current event loop, so this raises before the server can start.

This switches the synchronous entry point to asyncio.run(self.arun()), which creates and manages the loop for this one-shot runner. I also added a regression test that patches asyncio.get_event_loop() to raise with the Python 3.14-style failure and confirms run() still reaches the handler.

Refs #559.

Tests:

• .venv\Scripts\python -m pytest tests\test_server.py::test_run_creates_event_loop_when_none_exists -q
• .venv\Scripts\python -m pytest tests\test_server.py -q
• .venv\Scripts\python -m pytest -q
• .venv\Scripts\python -m mypy .
• git diff --check