[MM-57533] Send a warning when selective sync blocks a message

[JulienTant]

Summary

Send a warning when selective sync blocks a message + Add tests on the MessageWIllBePosted hook.

QA Steps

• Make sure Selective Sync is on
• Have 2 MSTeams users connected to Mattermost
• When user 1 tries to message user 2, an ephemeral message let them know that the message won't be sent to MS Teams

Ticket Link

https://mattermost.atlassian.net/browse/MM-57533

[JulienTant]

I want to make sure I understand your feedback correctly @lieut-data; do I leave my ephemeral message in place now until we pivot to unmuting Teams Primary<-->Teams Primary or do you think it's better not to implement it at all, and I clean this out of this PR but leave my other changes?

[lieut-data]

I want to make sure I understand your feedback correctly @lieut-data; do I leave my ephemeral message in place now until we pivot to unmuting Teams Primary<-->Teams Primary or do you think it's better not to implement it at all, and I clean this out of this PR but leave my other changes?

I think we ended up chatting 1:1 in Mattermost, but to close the loop: let's keep this for interactions between an MS Teams primary user with only synthetic users, as those won't sync, but otherwise remove.

[lieut-data]

@JulienTant, propose we keep just the unit test coverage from this change but otherwise revert any net new messages as we work towards defining next steps for the project.

[JulienTant]

WIll close this as the feature is not relevant and tests have been added since