Fix incorrect assert type in the AgentNode class (#22964)

api/core/workflow/nodes/agent/agent_node.py

     AgentInputTypeError,
     AgentInvocationError,
     AgentMessageTransformError,
+    AgentNodeError,
     AgentVariableNotFoundError,
     AgentVariableTypeError,
     ToolFileNotFoundError,
                     variables[variable_name] = variable_value
             elif message.type == ToolInvokeMessage.MessageType.FILE:
                 assert message.meta is not None
-                assert isinstance(message.meta, File)
+                assert isinstance(message.meta, dict)
+                # Validate that meta contains a 'file' key
+                if "file" not in message.meta:
+                    raise AgentNodeError("File message is missing 'file' key in meta")
+
+                # Validate that the file is an instance of File
+                if not isinstance(message.meta["file"], File):
+                    raise AgentNodeError(f"Expected File object but got {type(message.meta['file']).__name__}")
                 files.append(message.meta["file"])
             elif message.type == ToolInvokeMessage.MessageType.LOG:
                 assert isinstance(message.message, ToolInvokeMessage.LogMessage)