End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below
  Truncate agent history when context window is exceeded

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

This PR proposes a simple fix for the ContextWindowExceededError:

• when raised, truncate the context roughly in half
• save state and reload the truncated history when session is restored

Please note that this is not well tested, but it is ready for code review.

Link of any specific issues this addresses
Fix #4951
Fix #4894

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:2fcb088-nikolaik   --name openhands-app-2fcb088   docker.all-hands.dev/all-hands-ai/openhands:2fcb088