refactor: return copy of callbacks list to improve encapsulation

- Changed callbacks property to return a copy instead of direct reference
- Prevents external modification of internal pipeline state
- Addresses bot review feedback for better defensive programming

Author: jaywang172

src/google/adk/agents/callback_pipeline.py

@@ -139,12 +139,12 @@ def has_callbacks(self) -> bool:
 
   @property
   def callbacks(self) -> list[Callable]:
-    """Returns the list of callbacks in the pipeline.
+    """Returns a copy of the list of callbacks in the pipeline.
 
     Returns:
       List of callback functions.
     """
-    return self._callbacks
+    return self._callbacks.copy()
 
 
 def normalize_callbacks(