Fetch conversation for seed data tasks, minor model fixes (#485)

* Fetch conversation for seed data, fix models, remove redundant payload type checks

backend/test_data/generic/test_generic_data.json

@@ -54,5 +54,19 @@
     "parent_message_id": "cec432cf",
     "text": "I'm unsure how to interpret this. Is it a riddle?",
     "role": "assistant"
+  },
+  {
+    "task_message_id": "b8e98ed6",
+    "user_message_id": "89384709",
+    "parent_message_id": "0e276b98",
+    "text": "No, I just wanted to see how you reply when I type random characters. Can you tell me who invented Wikipedia?",
+    "role": "prompter"
+  },
+  {
+    "task_message_id": "9a0e7683",
+    "user_message_id": "6d452c57",
+    "parent_message_id": "0e276b98",
+    "text": "Sorry, my cat sat on my keyboard. Can you print a cat in ASCII art?",
+    "role": "prompter"
   }
 ]