Refine defensive checks based on code review feedback

- Remove StreamingChoices from type annotation since this is non-streaming
- Remove redundant type assertion as LiteLLM validates types
- Simplify tracing output with ternary expression

Author: gn00295120

src/agents/extensions/models/litellm_model.py

@@ -111,13 +111,12 @@ async def get_response(
             )
 
             message: litellm.types.utils.Message | None = None
-            first_choice: (
-                litellm.types.utils.Choices | litellm.types.utils.StreamingChoices | None
-            ) = None
+            first_choice: litellm.types.utils.Choices | None = None
             if response.choices and len(response.choices) > 0:
-                first_choice = response.choices[0]
-                assert isinstance(first_choice, litellm.types.utils.Choices)
-                message = first_choice.message
+                choice = response.choices[0]
+                if isinstance(choice, litellm.types.utils.Choices):
+                    first_choice = choice
+                    message = first_choice.message
 
             if _debug.DONT_LOG_MODEL_DATA:
                 logger.debug("Received model response")
@@ -160,8 +159,10 @@ async def get_response(
                 usage = Usage()
                 logger.warning("No usage information returned from Litellm")
 
-            if tracing.include_data() and message is not None:
-                span_generation.span_data.output = [message.model_dump()]
+            if tracing.include_data():
+                span_generation.span_data.output = (
+                    [message.model_dump()] if message is not None else []
+                )
             span_generation.span_data.usage = {
                 "input_tokens": usage.input_tokens,
                 "output_tokens": usage.output_tokens,