Previously, if you clicked on a user message to edit it, and then, while
the user message has the editor pending, sent a new message via the
textarea, the whole thread would be grayed out because we hadn't
dismissed the to-be-edited pending user message. That's now fixed.

Release Notes:

- agent: Fixed a bug that would make the whole thread be grayed out upon
sending a new message while a user message had a pending edit.