fix(login): Redirect to IDP flow when password auth is disabled (#10839)

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

Author: peintnermax