fix: prefer _full_model_name over _settings.model in tracing

The API-provided full model name is more specific than the
user-provided model name (e.g. includes version/snapshot details).
Reorder the lookup in _get_model_name and add a comment where the
Responses service sets the field.

src/pipecat/services/openai/responses/llm.py

@@ -292,6 +292,8 @@ class OpenAIResponsesLLMService(LLMService):
 
                     model = getattr(response, "model", None)
                     if model:
+                        # This field is used by @traced_llm for more detailed
+                        # model name in tracing spans
                         self._full_model_name = model
 
         # Process any function calls

src/pipecat/utils/tracing/service_decorators.py

@@ -51,8 +51,10 @@ def _get_model_name(service) -> str:
     check all the places we used to store it.
     """
     return (
-        getattr(getattr(service, "_settings", None), "model", None)
-        or getattr(service, "_full_model_name", None)
+        # Some services store an API-response-provided detailed "full" name,
+        # which is distinct from the user-provided model name
+        getattr(service, "_full_model_name", None)
+        or getattr(getattr(service, "_settings", None), "model", None)
         or getattr(service, "model_name", None)
         or getattr(service, "_model_name", None)
         or "unknown"