Fix test isolation: reset TraceCollectionListener singleton in cleanup_event_handlers fixture

Prevents singleton state pollution (trace_batch_id="debug-trace-batch") from
test_flow_conversation.py leaking into test_trace_enable_disable.py when both
files run in the same pytest-xdist worker under --dist=loadfile. The polluted
singleton caused a background HTTP request to fake.crewai.com that corrupted
VCR cassette state, preventing the OpenAI LLM entry from matching.

conftest.py

@@ -197,6 +197,21 @@ def cleanup_event_handlers() -> Generator[None, Any, None]:
     except Exception:  # noqa: S110
         pass
 
+    try:
+        from crewai.events.listeners.tracing.trace_listener import (
+            TraceCollectionListener,
+        )
+
+        if TraceCollectionListener._instance is not None:
+            instance_dict = TraceCollectionListener._instance.__dict__
+            if "_initialized" in instance_dict:
+                del TraceCollectionListener._instance._initialized
+            if "_listeners_setup" in instance_dict:
+                del TraceCollectionListener._instance._listeners_setup
+        TraceCollectionListener._instance = None
+    except Exception:  # noqa: S110
+        pass
+
 
 @pytest.fixture(autouse=True, scope="function")
 def reset_event_state() -> None: