fix: Propagate LiteLLM finish_reason to LlmResponse for use in callbacks

Closes #3114.

Co-authored-by: Eliza Huang <heliza@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3319 from lizzij:fix/litellm-finish-reason-3109 da6ed0aec514231fc274fee744f885ae95b9d45c
PiperOrigin-RevId: 825783229

tests/unittests/models/test_litellm.py

@@ -19,6 +19,7 @@ from unittest.mock import Mock
 import warnings
 
 from google.adk.models.lite_llm import _content_to_message_param
+from google.adk.models.lite_llm import _FINISH_REASON_MAPPING
 from google.adk.models.lite_llm import _function_declaration_to_tool_param
 from google.adk.models.lite_llm import _get_content
 from google.adk.models.lite_llm import _message_to_generate_content_response
@@ -1938,3 +1939,113 @@ def test_non_gemini_litellm_no_warning():
     # Test with non-Gemini model
     LiteLlm(model="openai/gpt-4o")
     assert len(w) == 0
+
+
+@pytest.mark.parametrize(
+    "finish_reason,response_content,expected_content,has_tool_calls",
+    [
+        ("length", "Test response", "Test response", False),
+        ("stop", "Complete response", "Complete response", False),
+        (
+            "tool_calls",
+            "",
+            "",
+            True,
+        ),
+        ("content_filter", "", "", False),
+    ],
+    ids=["length", "stop", "tool_calls", "content_filter"],
+)
+@pytest.mark.asyncio
+async def test_finish_reason_propagation(
+    mock_acompletion,
+    lite_llm_instance,
+    finish_reason,
+    response_content,
+    expected_content,
+    has_tool_calls,
+):
+  """Test that finish_reason is properly propagated from LiteLLM response."""
+  tool_calls = None
+  if has_tool_calls:
+    tool_calls = [
+        ChatCompletionMessageToolCall(
+            type="function",
+            id="test_id",
+            function=Function(
+                name="test_function",
+                arguments='{"arg": "value"}',
+            ),
+        )
+    ]
+
+  mock_response = ModelResponse(
+      choices=[
+          Choices(
+              message=ChatCompletionAssistantMessage(
+                  role="assistant",
+                  content=response_content,
+                  tool_calls=tool_calls,
+              ),
+              finish_reason=finish_reason,
+          )
+      ]
+  )
+  mock_acompletion.return_value = mock_response
+
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user", parts=[types.Part.from_text(text="Test prompt")]
+          )
+      ],
+  )
+
+  async for response in lite_llm_instance.generate_content_async(llm_request):
+    assert response.content.role == "model"
+    # Verify finish_reason is mapped to FinishReason enum
+    assert isinstance(response.finish_reason, types.FinishReason)
+    # Verify correct enum mapping using the actual mapping from lite_llm
+    assert response.finish_reason == _FINISH_REASON_MAPPING[finish_reason]
+    if expected_content:
+      assert response.content.parts[0].text == expected_content
+    if has_tool_calls:
+      assert len(response.content.parts) > 0
+      assert response.content.parts[-1].function_call.name == "test_function"
+
+  mock_acompletion.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_finish_reason_unknown_maps_to_other(
+    mock_acompletion, lite_llm_instance
+):
+  """Test that unknown finish_reason values map to FinishReason.OTHER."""
+  mock_response = ModelResponse(
+      choices=[
+          Choices(
+              message=ChatCompletionAssistantMessage(
+                  role="assistant",
+                  content="Test response",
+              ),
+              finish_reason="unknown_reason_type",
+          )
+      ]
+  )
+  mock_acompletion.return_value = mock_response
+
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user", parts=[types.Part.from_text(text="Test prompt")]
+          )
+      ],
+  )
+
+  async for response in lite_llm_instance.generate_content_async(llm_request):
+    assert response.content.role == "model"
+    # Unknown finish_reason should map to OTHER
+    assert isinstance(response.finish_reason, types.FinishReason)
+    assert response.finish_reason == types.FinishReason.OTHER
+
+  mock_acompletion.assert_called_once()