fix: fix typo in raise exception when tool is unsupported

PiperOrigin-RevId: 599869191

Author: vertex-sdk-bot

vertexai/generative_models/_generative_models.py

@@ -294,7 +294,7 @@ def _prepare_request(
                 elif isinstance(tool, Tool):
                     gapic_tools.append(tool._raw_tool)
                 else:
-                    raise TypeError("Unexpected tool type: {tool}.")
+                    raise TypeError(f"Unexpected tool type: {tool}.")
 
         return gapic_prediction_service_types.GenerateContentRequest(
             # The `model` parameter now needs to be set for the vision models.