feat: improve event bus thread safety and async support

Add thread-safe, async-compatible event bus with read–write locking and
handler dependency ordering. Remove blinker dependency and implement
direct dispatch. Improve type safety, error handling, and deterministic
event synchronization.

Refactor tests to auto-wait for async handlers, ensure clean teardown,
and add comprehensive concurrency coverage. Replace thread-local state
in AgentEvaluator with instance-based locking for correct cross-thread
access. Enhance tracing reliability and event finalization.

lib/crewai/tests/utils.py

@@ -0,0 +1,39 @@
+"""Test utilities for CrewAI tests."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+
+
+def wait_for_event_handlers(timeout: float = 5.0) -> None:
+    """Wait for all pending event handlers to complete.
+
+    This helper ensures all sync and async handlers finish processing before
+    proceeding. Useful in tests to make assertions deterministic.
+
+    Args:
+        timeout: Maximum time to wait in seconds.
+    """
+    from crewai.events.event_bus import crewai_event_bus
+
+    loop = getattr(crewai_event_bus, "_loop", None)
+
+    if loop and not loop.is_closed():
+
+        async def _wait_for_async_tasks() -> None:
+            tasks = {
+                t for t in asyncio.all_tasks(loop) if t is not asyncio.current_task()
+            }
+            if tasks:
+                await asyncio.gather(*tasks, return_exceptions=True)
+
+        future = asyncio.run_coroutine_threadsafe(_wait_for_async_tasks(), loop)
+        try:
+            future.result(timeout=timeout)
+        except Exception:  # noqa: S110
+            pass
+
+    crewai_event_bus._sync_executor.shutdown(wait=True)
+    crewai_event_bus._sync_executor = ThreadPoolExecutor(
+        max_workers=10,
+        thread_name_prefix="CrewAISyncHandler",
+    )