perf: reduce framework overhead — lazy event bus, skip tracing when disabled (#5187)

* perf: reduce framework overhead for NVIDIA benchmarks

- Lazy initialize event bus thread pool and event loop on first emit()
  instead of at import time (~200ms savings)
- Skip trace listener registration (50+ handlers) when tracing disabled
- Skip trace prompt in non-interactive contexts (isatty check) to avoid
  20s timeout in CI/Docker/API servers
- Skip flush() when no events were emitted (avoids 30s timeout waste)
- Add _has_pending_events flag to track if any events were emitted
- Add _executor_initialized flag for lazy init double-checked locking

All existing behavior preserved when tracing IS enabled. No public APIs
changed - only conditional guards added.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix: address PR review comments — tracing override, executor init order, stdin guard, unused import

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* style: fix ruff formatting in trace_listener.py and utils.py

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Iris Clawd <iris@crewai.com>
Co-authored-by: Greyson LaLonde <greyson.r.lalonde@gmail.com>