Update conversation metadata with cost and token metrics

[rbren]

This PR updates the conversation metadata to include cost/token metrics from events.

Changes

1. Added new fields to ConversationMetadata class:

   • accumulated_cost: Tracks the total cost of the conversation
   • prompt_tokens: Tracks the number of prompt tokens used
   • completion_tokens: Tracks the number of completion tokens used
   • total_tokens: Tracks the total number of tokens used

2. Renamed _update_timestamp_for_conversation to _update_conversation_for_event to better reflect its expanded purpose

3. Modified _create_conversation_update_callback to pass the event to the callback function

4. Updated the function to extract and store metrics from the event when available

All tests are passing with these changes.

---

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:cd25e32-nikolaik   --name openhands-app-cd25e32   docker.all-hands.dev/all-hands-ai/openhands:cd25e32

[enyst]

It looks like _accumulated_token_usage is already a TokenUsage object which includes the additions:
https://github.com/All-Hands-AI/OpenHands/pull/7511/files#diff-9010f8fab42fcc1b1464f428310e3ef8f93d07c22b6b481bedbbc5a0d864386aR132

[rbren]

good catch! reverting