add cancellation details arg to testing ActivityEnvironment cancel

Author: THardy98

temporalio/testing/_activity.py

@@ -76,14 +76,22 @@ def __init__(self) -> None:
         self._activities: Set[_Activity] = set()
         self.cancellation_details = None
 
-    def cancel(self) -> None:
+    def cancel(self, cancellation_details: Optional[temporalio.activity.ActivityCancellationDetails] = None) -> None:
         """Cancel the activity.
 
+        Args:
+            cancellation_details: Optional details about the cancellation. When provided, these 
+                will be accessible through temporalio.activity.cancellation_details() 
+                in the activity after cancellation.
+
         This only has an effect on the first call.
         """
         if self._cancelled:
             return
         self._cancelled = True
+        # Set cancellation details if provided
+        if cancellation_details is not None:
+            self.cancellation_details = cancellation_details
         for act in self._activities:
             act.cancel()