Closes#10671
# Which Problems Are Solved
Users with only password authentication method were immediately shown an
error "Username Password not allowed" when
`loginSettings.allowUsernamePassword` was set to false. However, the IDP
flow could potentially allow the user to register a new account or link
an existing account, providing a better user experience than a dead-end
error.
# How the Problems Are Solved
- Modified single password method case to attempt IDP redirect before
showing error
- This allows users to potentially register or link accounts through the
IDP flow instead of hitting an immediate error
- Only show error as last resort when no IDP alternative is available