fix: Fix race conditions in EventStream subscriber management

[malhotra5]

This PR fixes several issues with subscriber management in EventStream that could cause subscribers to continue receiving events after being unsubscribed.

Problem

The current implementation has several race conditions and thread safety issues:

1. Subscribers can continue receiving events after being unsubscribed
2. Thread pools are not shut down gracefully
3. Dictionary modifications during iteration can cause errors
4. Lack of proper synchronization in subscriber management

Solution

This PR adds several improvements:

1. Proper thread synchronization using locks
2. Changes cleanup order to prevent events after unsubscribe
3. Improves thread pool shutdown behavior
4. Adds snapshot mechanism for safe iteration
5. Adds better error handling and logging

Changes

• Added locks around subscribe/unsubscribe operations
• Changed cleanup order to remove subscribers before shutting down resources
• Added non-blocking thread pool shutdown
• Added snapshot mechanism for safe iteration over subscribers
• Added better error handling and debug logging
• Added comprehensive tests

Testing

Added several new test cases:

• test_subscriber_behavior: Basic subscriber functionality
• test_subscriber_stress: Stress testing with rapid subscribe/unsubscribe
• test_subscriber_unsubscribe_bug: Specific bug reproduction
• test_subscriber_unsubscribe_concurrent: Concurrent operations
• test_subscriber_race_condition: Race condition testing

All tests pass and demonstrate proper handling of:

• Rapid subscribe/unsubscribe operations
• Concurrent event processing
• Thread pool lifecycle
• Race conditions
• Edge cases and error conditions

Notes

This is a draft PR to get early feedback on the approach. The changes are significant but necessary to fix the underlying issues.

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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:612c6ee-nikolaik   --name openhands-app-612c6ee   docker.all-hands.dev/all-hands-ai/openhands:612c6ee