_ensure_team_member silently swallowed every exception from
add_user_to_team, assuming failure meant the user was already a
member. Real failures (bad permissions, domain restrictions, wrong
team ID) were masked and only surfaced later as a confusing "No team
member found" error when adding the user to a channel. Now checks
actual membership via get_team_member and raises a clear
MattermostError on genuine add failures.
