Agent session no longer stuck in starting on raised exception

[tofarr]

Now making sure that the starting flag is no longer left in a True state if an error is thrown while starting

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

---

Improves agent session error handling in two ways:

1. Ensures the starting flag is properly reset when an exception occurs during startup, preventing the session from getting stuck in a "starting" state
2. Adds proper error handling for unknown errors from start_or_attach_to_runtime, ensuring HTTP sessions are properly closed in all error cases

These changes prevent unnecessary waits during local shutdown and improve the overall robustness of the session management.

---

Link of any specific issues this addresses

---

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:0f34af3-nikolaik   --name openhands-app-0f34af3   docker.all-hands.dev/all-hands-ai/openhands:0f34af3