fix: prevent orphaned threads in agent timeout handling

[Rakshit-gen]

Fixes #4135

• Replace context manager with explicit try/finally block
• Check cancel() return value and handle both success/failure cases
• Always call executor.shutdown(wait=True) to ensure thread cleanup
• Prevents resource leaks and thread accumulation under load

Fixes issue where tasks timing out would create orphaned threads that hold resources indefinitely, leading to memory leaks and potential crashes in production environments.

---

Note

Refactors Agent._execute_with_timeout to robustly cancel/wait on futures and always shutdown the ThreadPoolExecutor, preventing orphaned threads and resource leaks.

• Agent execution (sync timeout path):
  • Replace context manager with explicit ThreadPoolExecutor and finally-ensured executor.shutdown(wait=True).
  • On timeout and other exceptions, check future.cancel(); if already running, wait briefly on future.result(...) to ensure thread cleanup.
  • Preserve behavior by raising TimeoutError/RuntimeError while preventing orphaned threads.

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