Fixes#39189.
`IsUserBlockedBy` intentionally treats admin users as not blocked, but
`CanUnblockUser` was also using it to determine whether a blocking
relationship exists. If a previously blocked user is later promoted to
admin, the existing `user_blocking` record remains but can no longer be
removed.
This change separates those two concerns by adding `HasBlocking` for
checking the persisted blocking relationship. `CanUnblockUser` uses that
relationship check while `IsUserBlockedBy` keeps its existing admin-user
behavior.
A regression test verifies that an admin is still not considered blocked
while an existing blocking relationship can still be unblocked.
