Update AIService.set_model_name to AIService._sync_model_name_to_metrics to:

- indicate clearly that it's not meant for public use
- make it clear the `self._settings` is the single source of truth for model information
- set the stage for an upcoming change where `AIService` subclasses won't have to ever worry about explicitly calling an `AIService` method to sync model name to metrics

Across all services, switch from accessing `self._model_name` or `self.model_name` in favor of `self._settings.model`.

src/pipecat/services/fal/image.py

@@ -78,7 +78,8 @@ class FalImageGenService(ImageGenService):
             **kwargs: Additional arguments passed to parent ImageGenService.
         """
         super().__init__(**kwargs)
-        self.set_model_name(model)
+        self._settings.model = model
+        self._sync_model_name_to_metrics()
         self._params = params
         self._aiohttp_session = aiohttp_session
         if key:
@@ -103,7 +104,7 @@ class FalImageGenService(ImageGenService):
         logger.debug(f"Generating image from prompt: {prompt}")
 
         response = await fal_client.run_async(
-            self.model_name,
+            self._settings.model,
             arguments={"prompt": prompt, **self._params.model_dump(exclude_none=True)},
         )