fix: skip signal handler registration in non-main thread

[Bharath-designer]

Improve telemetry behavior when initialized from non-main threads.

• Skip signal handler registration when Telemetry is created outside the main thread to avoid noisy ValueError tracebacks.
• Expand the debug log message to explain how to disable telemetry via CREWAI_DISABLE_TELEMETRY and link to the telemetry docs.
• Update telemetry tests to cover the new log message contents and non-main-thread behavior.

Fixes: #4289

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Note

Improve telemetry robustness in threaded contexts

• Update telemetry._register_shutdown_handlers to detect non‑main threads and skip calling '_register_signal_handler'; logs a debug message mentioning CREWAI_DISABLE_TELEMETRY=true and linking to https://docs.crewai.com/telemetry to avoid noisy ValueError tracebacks.
• Add tests in tests/telemetry/test_telemetry.py covering: skip on non‑main thread (no ValueError), presence of the debug log message, and that main thread still registers handlers; minor test import adjustment.

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