Send-to-channel protection doesn't work

The bot used to un-send-to-channel messages like this (and the user would get an ephemeral msg explaining what's up)

At some point this seemed to stop working? It used to very occasionally not work, but now it always doesn't work.

When fixing this, be sure to test the "send to channel" checkbox, and also sending to channel after the message has been sent. And add logs for when a message is un-sent-to-channel.

Start by tracing the send-to-channel handling for both the checkbox path and sending to channel after the message has been posted. Reproduce both cases, verify the message is unsent and the user receives the ephemeral explanation, then add logs for each unsend event and tests covering both behaviors.

Written by the indexing model from the issue text.