fix(telemetry): skip signal registration on non-main threads

[thecaptain789]

Adds a main thread check before attempting to register signal handlers in the telemetry module.

Signal handlers can only be registered from the main thread, and attempting to do so from other threads raises ValueError with noisy tracebacks that clutter the console output.

This commonly occurs when CrewAI is imported from web frameworks like Streamlit, Flask, Django, or Jupyter notebooks which may initialize imports from background threads.

The fix adds a simple check at the start of _register_shutdown_handlers():

if threading.current_thread() is not threading.main_thread():
    logger.debug(
        "Skipping signal handler registration (not running in main thread)"
    )
    return

The atexit handler is still registered since it works from any thread.

Fixes #4289

---

Note

Low Risk
Small, localized change in shutdown/signal setup that mainly avoids exceptions in non-main-thread environments; it should not affect telemetry behavior in normal main-thread execution.

Overview
Prevents noisy ValueError tracebacks during telemetry initialization by skipping OS signal handler registration when not running on the main thread.

Telemetry._register_shutdown_handlers() still registers the atexit shutdown hook, but now early-returns (with a debug log) before calling signal.signal(...) when imported/initialized from background threads (common in web/notebook environments).

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