Previously, editing a past user message in Agents chat waited for the PATCH round-trip and follow-up refetch before the transcript tail visibly changed. Even after the initial optimistic update, the edited bubble could still flicker away while the UI reconciled from the old user-message ID to the new server-assigned replacement ID.

Keep message editing optimistic end-to-end: update the edited user bubble and truncate the tail immediately, carry that state through the authoritative replacement message until the cache catches up, and roll back the visible draft cleanly on failure. This also centralizes edited-submission request/render preparation and adds focused regression coverage around the chat store bridge, rollback behavior, and attachment/draft restoration.