fix: skip signal handler registration in non-main thread

[devin-ai-integration]

Summary

Fixes #4289 - When CrewAI is initialized from a non-main thread (e.g., in Streamlit, Flask, Django, Jupyter), the telemetry module was printing multiple noisy ValueError tracebacks for each signal handler registration attempt.

The fix adds a proactive main thread check in _register_shutdown_handlers() before attempting signal registration. If not in the main thread, a debug message is logged and signal handler registration is skipped entirely, avoiding the noisy exception tracebacks.

Changes:

• Added thread check using threading.current_thread() is not threading.main_thread()
• Log a debug message when skipping signal handler registration
• Added 3 tests covering: signal handlers skipped in non-main thread, debug message logged, and signal handlers still registered in main thread

Review & Testing Checklist for Human

• Verify the fix end-to-end: Run the reproduction script from the issue to confirm no more noisy tracebacks:

  import threading
  def run_crewai_in_thread():
      from crewai import Agent
      print("CrewAI imported successfully in thread")
  thread = threading.Thread(target=run_crewai_in_thread)
  thread.start()
  thread.join()

• Confirm logger.debug() is the appropriate log level (issue suggested logger.warning() with docs link, but debug is less noisy which aligns with the goal)
• Verify signal handlers still work correctly when running from main thread (normal usage)

Notes

• Link to Devin run: https://app.devin.ai/sessions/cd7611ac0a6d4404b84b541f433278f1
• Requested by: João (joao@crewai.com)

[devin-ai-integration]

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[Bharath-designer]

I'd suggest enhancing the log message to be more actionable for users. The current message doesn't tell them how to disable the warning or where to learn more.

Suggested change:

logger.debug(
            "CrewAI telemetry: Skipping signal handler registration (not running in main thread). "
            "To disable this warning or disable crewai telemetry entirely, set CREWAI_DISABLE_TELEMETRY=true. "
            "See: https://docs.crewai.com/telemetry"
        )

[Bharath-designer]

Thanks, but I'd like to submit this fix myself as my first open source contribution.
I'll create a separate PR with an improved implementation.