docs: address Gemini Code Assist bot review feedback

- Update llm_response.py docstring to use 'underlying model providers'
  instead of specifically 'LiteLLM' for better generalization

- Add explanatory comment for tool_calls->STOP mapping:
  - FinishReason.TOOL_CALL enum does not exist in google-genai
  - Tool calls are normal completion (STOP is semantically correct)
  - Consistent with Gemini native behavior (lite_llm.py:910)

Addresses bot feedback:
- Make docstring more general about finish_reason sources
- Clarify why tool_calls maps to STOP (not a different enum)

Author: aperepel

src/google/adk/models/lite_llm.py

@@ -65,11 +65,15 @@
 _EXCLUDED_PART_FIELD = {"inline_data": {"data"}}
 
 # Mapping of LiteLLM finish_reason strings to FinishReason enum values
+# Note: tool_calls/function_call map to STOP because:
+# 1. FinishReason.TOOL_CALL enum does not exist (as of google-genai 0.8.0)
+# 2. Tool calls represent normal completion (model stopped to invoke tools)
+# 3. Gemini native responses use STOP for tool calls (see lite_llm.py:910)
 _FINISH_REASON_MAPPING = {
     "length": types.FinishReason.MAX_TOKENS,
     "stop": types.FinishReason.STOP,
-    "tool_calls": types.FinishReason.STOP,
-    "function_call": types.FinishReason.STOP,
+    "tool_calls": types.FinishReason.STOP,  # Normal completion with tool invocation
+    "function_call": types.FinishReason.STOP,  # Legacy function call variant
     "content_filter": types.FinishReason.SAFETY,
 }
 

src/google/adk/models/llm_response.py

@@ -81,8 +81,8 @@ class LlmResponse(BaseModel):
   finish_reason: Optional[types.FinishReason] = None
   """The finish reason of the response.
 
-  Always a types.FinishReason enum. LiteLLM string values are mapped to
-  corresponding enum values (with fallback to OTHER for unknown values).
+  Always a types.FinishReason enum. String values from underlying model providers
+  are mapped to corresponding enum values (with fallback to OTHER for unknown values).
   """
 
   error_code: Optional[str] = None