fix: Safer fix for UI widget merging in ADK

Revert global list extension in deep_merge_dicts and instead explicitly handle list extension only for render_ui_widgets in merge_parallel_function_response_events.

Co-authored-by: Vishwa Murugan <vishwamurugan@google.com>
PiperOrigin-RevId: 891147765

Author: vishwa180

src/google/adk/flows/llm_flows/functions.py

@@ -1151,12 +1151,10 @@ def __build_response_event(
 
 
 def deep_merge_dicts(d1: dict, d2: dict) -> dict:
-  """Recursively merges d2 into d1, extending lists where keys collide."""
+  """Recursively merges d2 into d1."""
   for key, value in d2.items():
     if key in d1 and isinstance(d1[key], dict) and isinstance(value, dict):
       d1[key] = deep_merge_dicts(d1[key], value)
-    elif key in d1 and isinstance(d1[key], list) and isinstance(value, list):
-      d1[key].extend(value)
     else:
       d1[key] = value
   return d1
@@ -1181,14 +1179,25 @@ def merge_parallel_function_response_events(
 
   # Merge actions from all events
   merged_actions_data: dict[str, Any] = {}
+  aggregated_ui_widgets = []
   for event in function_response_events:
     if event.actions:
+      actions_dict = event.actions.model_dump(exclude_none=True, by_alias=True)
+      ui_widgets = actions_dict.pop(
+          'renderUiWidgets', None
+      ) or actions_dict.pop('render_ui_widgets', None)
+      if ui_widgets:
+        aggregated_ui_widgets.extend(ui_widgets)
+
       # Use `by_alias=True` because it converts the model to a dictionary while respecting field aliases, ensuring that the enum fields are correctly handled without creating a duplicate.
       merged_actions_data = deep_merge_dicts(
           merged_actions_data,
-          event.actions.model_dump(exclude_none=True, by_alias=True),
+          actions_dict,
       )
 
+  if aggregated_ui_widgets:
+    merged_actions_data['renderUiWidgets'] = aggregated_ui_widgets
+
   merged_actions = EventActions.model_validate(merged_actions_data)
 
   # Create the new merged event