Switch from context manager to a straight binding to avoid deadlock

[blast-hardcheese]

Why

Attempting to run uv run pytest -k test_ignore_flood_subscription hangs for a significant amount of time.

This is because in __aexit__ we await on the server's wait_closed(), which never completes because we still haven't called server.close():

    async def __aexit__(...) -> None:
        self.ws_server.close()
        await self.ws_server.wait_closed()

An alternate approach would be to wrap the entire innards of the async context in a try: ... finally: server.close().

What changed

Reflowed clientserver text fixture to avoid a deadlock.

Test plan

CI