Fix error when getting messages from DB in chat_app example (#488)

Author: DurandA

pydantic_ai_examples/chat_app.py

@@ -31,6 +31,7 @@
 from pydantic_ai.messages import (
     ModelMessage,
     ModelMessagesTypeAdapter,
+    ModelRequest,
     ModelResponse,
     TextPart,
     UserPromptPart,
@@ -86,14 +87,15 @@ class ChatMessage(TypedDict):
 
 
 def to_chat_message(m: ModelMessage) -> ChatMessage:
-    if isinstance(m, UserPromptPart):
-        return {
-            'role': 'user',
-            'timestamp': m.timestamp.isoformat(),
-            'content': m.content,
-        }
+    first_part = m.parts[0]
+    if isinstance(m, ModelRequest):
+        if isinstance(first_part, UserPromptPart):
+            return {
+                'role': 'user',
+                'timestamp': first_part.timestamp.isoformat(),
+                'content': first_part.content,
+            }
     elif isinstance(m, ModelResponse):
-        first_part = m.parts[0]
         if isinstance(first_part, TextPart):
             return {
                 'role': 'model',