refactor: move session initialization from WebSocket to REST API

[rbren]

Currently the frontend starts a session by connecting to the websocket and then sending an INIT event with initialization data. This PR changes that to use a REST API endpoint instead.

Changes

• Add POST /api/conversation endpoint for session initialization
• Update frontend to use new endpoint instead of WebSocket INIT event
• Remove WebSocket INIT event handling from backend
• Move all conversation-related routes (e.g. list-files and submit-feedback) to /api/conversation/{id}/...
• Main frontend now includes conversation ID in the URL
• Removed all the JWT token encryption logic
  • SIDs (now conversation IDs) are no longer secret
  • This PR CANNOT BE MERGED until [Refactor]: Changes to Github Authentication #5371 is merged, and a new auth mechanism is put in place here, at least for multi-tenant
    • Auth mechanism is in place now
  • TBD if we need auth for OSS, since everything is on OSS--would love to hear any opinions here
    • I've explicitly mentioned that OSS is meant to be run locally now, with no isolation

Resolves #4281

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:9e9e308-nikolaik   --name openhands-app-9e9e308   docker.all-hands.dev/all-hands-ai/openhands:9e9e308

[tofarr]

The good:

• I like the conversation_id in the URL (This makes so many things in the SAAS env easier) - the URL may be shorter if we used base62 vs hex for the URL (22 vs 32 chars - SocketIO does this too.)
• I Love the appropriate use of contexts for conversations in the frontend 🙌
• Love the removal of github_auth from the OSS api! 🙌
• The new names make a lot of sense (start_local_session -> start_agent_loop, init_or_join_session -> join_conversation 🙌)

The Questions:

• (Noted inline mostly)
• The first time I loaded I got an odd blinking effect when initializing the session - I was not able to reproduce this on subsequent attempts.

[amanape]

The first time I loaded I got an odd blinking effect when initializing the session - I was not able to reproduce this on subsequent attempts.

If you've recently rebuilt the frontend, Vite does some dependency optimizations which causes some refreshes. It is a first time (per fresh build) only thing

[amanape]

I have run the app again with the latest commits and it seems to be working OK. I was only able to replicate the same error message I got earlier by updating the :conversationId param in the URL directly to simulate accessing a "non-existent" conversation. I assume this is expected though since I am going to handle this in #5376

LGTM!

[rbren]

OK all changes made, and things seem to be working well! 🚢

[diwu-sf]

This is a good change, we've been wanting something like this too vs the custom websocket init handshake process.