Fixed an issue that start_callback was not invoked for some LLM services.

src/pipecat/services/ai_services.py

@@ -175,6 +175,7 @@ class LLMService(AIService):
             f = self._callbacks[None]
         else:
             return None
+        await self.call_start_function(context, function_name)
         await context.call_function(
             f,
             function_name=function_name,

src/pipecat/services/openai.py

@@ -266,7 +266,6 @@ class BaseOpenAILLMService(LLMService):
                 if tool_call.function and tool_call.function.name:
                     function_name += tool_call.function.name
                     tool_call_id = tool_call.id
-                    await self.call_start_function(context, function_name)
                 if tool_call.function and tool_call.function.arguments:
                     # Keep iterating through the response to collect all the argument fragments
                     arguments += tool_call.function.arguments