Disable authentication with auto-generated passwords

When registering via OAuth provider, an auto-generated password is set for the user. We do this to maintain data consistency.

Passwords are also required for git+HTTP operations. However, until the user does not set their own password, there's no way for them to retrieve / use the current one.

Since these passwords are not usable in any way, let's disable authentication with them until the password is manually set.

This would also helped avoid https://gitlab.com/gitlab-com/gl-security/security-operations/sirt/operations/-/issues/2167.

Availability & Testing

Add lower level test to ensure the auto generated password is not usable to sign in and perform Git operations.