Add return annotation to get_model_signature

get_model_signature is intended as a method for obtaining the call signature of get_resolution_function, and therefore its return annotation should be a subclass of InstrumentModel

src/resolution_functions/instrument.py

@@ -602,7 +602,7 @@ def get_model_signature(self, model_name: Optional[str] = None) -> Signature:
 
             params[key] = value.replace(**args, kind=Parameter.KEYWORD_ONLY)
 
-        return Signature(parameters=list(params.values()))
+        return Signature(parameters=list(params.values()), return_annotation=model_class)
 
     @property
     def available_models(self) -> list[str]: