Feat - browser client can now close sessions.

[tofarr]

**Client can now instruct server to delete a running session. Added framework for commanding that a session stops. **

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

This PR fixes an issue where conversations could not be deleted while they were running. The changes include:

• Modified delete_conversation to properly close the session when attempting to delete a running conversation instead of returning False
• Added close_session method to SessionManager to handle session closing across the cluster
• Refactored session closing logic to handle both local and remote session closures
• Added handling for close_session message type in the session manager

Testing

To test:

1. Start a conversation and let it run
2. Try to delete the conversation while it is running
3. Verify that the conversation is properly deleted and the session is closed
4. Verify that this works across multiple nodes in a cluster setup

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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:158fee9-nikolaik   --name openhands-app-158fee9   docker.all-hands.dev/all-hands-ai/openhands:158fee9