Message deletion now preserves conversation branches

What changed

First-turn messages can now be deleted with the same splice behavior as other messages. Deleting a multi-model reply group removes only the assistant replies while preserving the user's question and later messages. Single-message, multi-model group, and multi-select deletion are unavailable while the affected assistant reply is still generating.

Why this matters to the user

Deleting messages no longer removes an entire conversation subtree when the action represents only a multi-model reply group.

What the user should do

Nothing — automatic.

Notes for release manager

The multi-model group deletion confirmation copy was updated to describe the preserved messages. The new message.delete.generating_unavailable copy explains why deletion is disabled during generation.