fix: handle non-dict history/messages in chat_message migration (#22588)

Some databases contain chat records where 'history' or 'messages' are
stored as lists instead of dicts. This causes an AttributeError
('list' object has no attribute 'items') during the
8452d01d26d7_add_chat_message_table migration.

Add isinstance checks to skip chat records with unexpected data shapes
gracefully, matching the existing pattern used for individual message
validation.

backend/open_webui/migrations/versions/8452d01d26d7_add_chat_message_table.py

@@ -151,7 +151,12 @@ def upgrade() -> None:
                 continue
 
         history = chat_data.get("history", {})
+        if not isinstance(history, dict):
+            continue
+
         messages = history.get("messages", {})
+        if not isinstance(messages, dict):
+            continue
 
         for message_id, message in messages.items():
             if not isinstance(message, dict):