Add summarize conversation feature

[xingyaoww]

• This change is worth documenting at https://docs.all-hands.dev/
• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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

This PR adds a new "Summarize conversation" button to the chat interface, allowing users to quickly get a summary of their conversation with the AI assistant. The summary is generated using the existing LLM summarizing condenser and is displayed as a system message in the chat.

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

This PR implements a summarize conversation feature in OpenHands by:

1. Adding a new summarize button next to the export trajectory button in the UI
2. Creating a new API endpoint in the backend to handle summarization requests
3. Using the existing LLMSummarizingCondenser to generate the summary
4. Displaying the summary as a system message in the chat interface

The implementation leverages the existing condenser implementation without modifying the agent controller or runtime. This approach allows for a clean implementation with minimal changes to the codebase.

Note: This implementation does not accumulate the LLM cost in the agent controller, which can be addressed in a future iteration.

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Link of any specific issues this addresses.

N/A

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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:e6d76bb-nikolaik   --name openhands-app-e6d76bb   docker.all-hands.dev/all-hands-ai/openhands:e6d76bb