[BUG] Message tree bug after Editing and Regenerating

**Describe the bug**
If you perform the following operations, the message tree will not be displayed correctly:

**To Reproduce**
- Send a message in a new chat
- Regenerate (the first message becomes 2/2)
- Send another message (the second message is created)
- Edit the first message
- The first message should become 3/3, but the second message becomes 2/2 instead.

**Screenshots**
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**Additional context**
Add any other context about the problem here.

Reproduce the reported sequence in the chat UI: send, regenerate, send again, then edit the first message. Trace how the message tree updates its position counts after regeneration and editing; done means the edited first message shows 3/3 while the second message retains the correct count.

Written by the indexing model from the issue text.