feat: map LiteLLM finish_reason strings to FinishReason enum

- Map finish_reason strings to proper FinishReason enum values in lite_llm.py
  - 'length' -> FinishReason.MAX_TOKENS
  - 'stop' -> FinishReason.STOP
  - 'tool_calls'/'function_call' -> FinishReason.STOP
  - 'content_filter' -> FinishReason.SAFETY
  - unknown values -> FinishReason.OTHER

- Add clarifying comment in tracing.py for string fallback path

- Update test_litellm.py to verify enum mapping:
  - Assert finish_reason is FinishReason enum instance
  - Verify correct enum values for each finish_reason string
  - Add test for unknown finish_reason mapping to OTHER

Benefits:
- Type consistency with Gemini native responses
- Avoids runtime warnings from string finish_reason
- Enables proper instanceof checks in callbacks
- Better integration with ADK telemetry

Author: aperepel

src/google/adk/models/lite_llm.py

@@ -505,7 +505,21 @@ def _model_response_to_generate_content_response(
 
   llm_response = _message_to_generate_content_response(message)
   if finish_reason:
-    llm_response.finish_reason = finish_reason
+    # Map LiteLLM finish_reason strings to FinishReason enum
+    # This provides type consistency with Gemini native responses and avoids warnings
+    finish_reason_str = str(finish_reason).lower()
+    if finish_reason_str == "length":
+      llm_response.finish_reason = types.FinishReason.MAX_TOKENS
+    elif finish_reason_str == "stop":
+      llm_response.finish_reason = types.FinishReason.STOP
+    elif "tool" in finish_reason_str or "function" in finish_reason_str:
+      # Handle tool_calls, function_call variants
+      llm_response.finish_reason = types.FinishReason.STOP
+    elif finish_reason_str == "content_filter":
+      llm_response.finish_reason = types.FinishReason.SAFETY
+    else:
+      # For unknown reasons, use OTHER
+      llm_response.finish_reason = types.FinishReason.OTHER
   if response.get("usage", None):
     llm_response.usage_metadata = types.GenerateContentResponseUsageMetadata(
         prompt_token_count=response["usage"].get("prompt_tokens", 0),

src/google/adk/telemetry/tracing.py

@@ -306,6 +306,7 @@ def trace_call_llm(
     if isinstance(llm_response.finish_reason, types.FinishReason):
       finish_reason_str = llm_response.finish_reason.name.lower()
     else:
+      # Fallback for string values (should not occur with LiteLLM after enum mapping)
       finish_reason_str = str(llm_response.finish_reason).lower()
     span.set_attribute(
         'gen_ai.response.finish_reasons',

tests/unittests/models/test_litellm.py

@@ -1967,11 +1967,58 @@ async def test_finish_reason_propagation(
 
   async for response in lite_llm_instance.generate_content_async(llm_request):
     assert response.content.role == "model"
-    assert response.finish_reason == finish_reason
+    # Verify finish_reason is mapped to FinishReason enum, not raw string
+    assert isinstance(response.finish_reason, types.FinishReason)
+    # Verify correct enum mapping
+    if finish_reason == "length":
+      assert response.finish_reason == types.FinishReason.MAX_TOKENS
+    elif finish_reason == "stop":
+      assert response.finish_reason == types.FinishReason.STOP
+    elif finish_reason == "tool_calls":
+      assert response.finish_reason == types.FinishReason.STOP
+    elif finish_reason == "content_filter":
+      assert response.finish_reason == types.FinishReason.SAFETY
     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()
+