Refactor EventStream to use single-file storage

[tofarr]

Event Stream Storage Refactoring

Changes

• Modified EventStream to store all events in a single JSON file per session instead of one file per event
• Added performance tests to verify the implementation
• Improved storage efficiency for small events (< 200 bytes)
• Added metrics for read/write operations

Performance Metrics

The performance tests show:

• Fast write operations (~0.35ms per event)
• Efficient read operations
• Reduced storage overhead by using a single file

Testing

• Added comprehensive performance tests in tests/load/test_event_stream_perf.py
• Tests verify event creation, storage, retrieval, and content integrity
• Includes metrics for common operations

Benefits

• Reduced filesystem overhead
• Better storage efficiency for small events
• Simpler file management
• Potentially better performance for bulk operations

Trade-offs

• Entire events list needs to be loaded into memory
• Potential file contention with multiple writers
• File size grows over time as more events are added

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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=ghcr.io/all-hands-ai/runtime:d96477a-nikolaik   --name openhands-app-d96477a   ghcr.io/all-hands-ai/runtime:d96477a