Fix #7105: Expose metrics directly on conversation rather than inside events

[enyst]

This PR fixes issue #7105 by implementing the reviewer suggestion to expose metrics directly from the conversation/state rather than reconstructing from events.

Changes:

• Added get_metrics() method to Conversation class that retrieves metrics directly from runtime.state.metrics
• Updated get_conversation_metrics() endpoint to try runtime state metrics first, falling back to event_stream.get_metrics()
• Added fallback mechanism for backward compatibility
• Updated tests to verify both direct state metrics and fallback to event_stream metrics

Tests:

• Added test for fallback behavior
• Updated existing tests to verify metrics are retrieved from runtime state first
• All tests pass with the new implementation

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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:9a8d6f1-nikolaik   --name openhands-app-9a8d6f1   docker.all-hands.dev/all-hands-ai/openhands:9a8d6f1