Premature AI reply display under old branch upon branching

[xmoiduts]

Editing an existing message (①) should create a new branch first, and then the reply can be generated under the new branch like (②).

In the current version, editing a message and request completion will result in the reply being generated under the current branch rather than the new one, perceived as the chat not being branched properly (③), then the reply is moved to the new branch after generation.