refactor(s08): use separate locks for tasks and notification queue

[JIA-ZI-LONG]

Summary

For real-world agent scenarios with high concurrency requirements, use dedicated locks for BackgroundManager data members:

• _tasks_lock: protects self.tasks dictionary
• _queue_lock: protects _notification_queue

This fine-grained lock design provides:

• Avoids blocking notification queue operations when querying task status
• Supports high-frequency notification writes when multiple background tasks complete simultaneously
• Reduces lock contention and improves overall throughput

Additional benefits:

• Establishes a consistent pattern of acquiring lock before write operations on data members
• Safer notification handling: append/clear operations on the notification queue are properly synchronized

Test plan

• Run python agents/s08_background_tasks.py to manually test background task functionality
• Verify correctness of task status reads and notification queue operations in multi-threaded scenarios

[vercel]

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[JIA-ZI-LONG]

If you decide to cherry-pick or apply this change manually, would you mind preserving the original author information? Thanks!