deadlock patch

[jackyzha0]

Why

deadlock in the server-side reconnect case:

1. establish handshake acquires session lock:

   river-python/src/replit_river/server_transport.py

   Line 249 in 6e2bc1a

   async with self._session_lock:

2. if there is a session mismatch, we close the old session:

   river-python/src/replit_river/server_transport.py

   Line 290 in 6e2bc1a

   await old_session.close()

3. session.close calls _close_session_callback

   river-python/src/replit_river/session.py

   Line 299 in 6e2bc1a

   await self._close_session_callback(self)

4. _delete_session also tries to acquire the session lock

   river-python/src/replit_river/server_transport.py

   Line 316 in 6e2bc1a

   async with self._session_lock:

What changed

1. dont need to call _delete_session as .close will already do that
2. lift out session close outside of session lock
3. in the handshake case, let the final call to get_or_create_session replace the session and close the old one

Test plan

added a test

Notes

I have a draft of a more in-depth approach on this branch which uses a lock-ownership-transfer based approach that should catch it more generically but ran into ownership problems lol

Seeing as we are planning on migrating chat service to Node anyways so we only have one River server implementation, we hopefully don't have to maintain this surface for much longer 🤞