fix: Recognize function responses as non-empty parts in LiteLLM

The `_part_has_payload` helper now returns False for parts containing a function response, preventing the addition of fallback user content when a user turn consists solely of tool outputs.

Close #4249

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 863319497

Author: GWeale

src/google/adk/models/lite_llm.py

@@ -497,6 +497,8 @@ def _part_has_payload(part: types.Part) -> bool:
     return True
   if part.file_data and (part.file_data.file_uri or part.file_data.data):
     return True
+  if part.function_response:
+    return True
   return False
 
 

tests/unittests/models/test_litellm.py

@@ -24,6 +24,7 @@
 from unittest.mock import Mock
 import warnings
 
+from google.adk.models.lite_llm import _append_fallback_user_content_if_missing
 from google.adk.models.lite_llm import _content_to_message_param
 from google.adk.models.lite_llm import _FILE_ID_REQUIRED_PROVIDERS
 from google.adk.models.lite_llm import _FINISH_REASON_MAPPING
@@ -990,6 +991,28 @@ async def test_generate_content_async_adds_fallback_user_message(
   )
 
 
+def test_append_fallback_user_content_ignores_function_response_parts():
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user",
+              parts=[
+                  types.Part.from_function_response(
+                      name="add", response={"result": 6}
+                  )
+              ],
+          )
+      ]
+  )
+
+  _append_fallback_user_content_if_missing(llm_request)
+
+  assert len(llm_request.contents) == 1
+  assert len(llm_request.contents[0].parts) == 1
+  assert llm_request.contents[0].parts[0].function_response is not None
+  assert llm_request.contents[0].parts[0].text is None
+
+
 litellm_append_user_content_test_cases = [
     pytest.param(
         LlmRequest(