Why

We've had persistent timeout errors in AI-Infra, and I suspect that it's related to not handling bumps in the connection correctly.

What changed

• WebSocket drops or send failures left _streams populated, so any in-flight RPC hung until the session fully shut down. That meant clients didn’t see an abort signal and could block indefinitely even though the transport was already defunct.
• Added _abort_all_streams() in src/replit_river/session.py#L289 and call it from both client_session.serve() and server_session.serve() on ConnectionClosed, FailedSendingMessageException, or any other unexpected exception (src/replit_river/client_session.py#L95, src/replit_river/server_session.py#L82). This immediately closes every active channel and clears _streams, ensuring callers are notified right away when the socket dies so they can retry or surface an error.

Test plan

CI/CD, ran against an internal branch with no issues 3x without flake.