Outbox message won't be saved unless the send button is clicked

[st3iny]

Steps to reproduce

1. Have a message in the outbox.
2. Click on it to edit it.

Case 1

1. Wait some time for it to be saved
2. Observe error message in bottom left corner of the composer: Error saving draft

Case 2

1. Close the modal or click outside.
2. Observe toast: Failed to save message

Case 3

1. Wait some time until the draft was saved.
2. Close the modal now.
3. Observe toast: Message saved
4. Open the same message again.
5. Observe that changes have been lost.

Changes are lost in all three cases.

Expected behavior

My changes should always be saved regardless if I press send again. That should be the case for the debounced background save as well as when closing/minimizing the composer.

Actual behavior

My changes are never saved unless I click the send button manually.

Mail app version

4.1.2 (also main)

Additional info

Could be a regression from #8469.

Requires #10754 (in order to be able to actually edit messages)