OAuth: Fix age validation edge case

[islemaster]

I believe this will fix this Honeybadger error.

ActiveRecord::RecordInvalid in api # import_google_classroom
Validation failed: Student user age is not included in the list

This happens for both Google Classroom and Clever.

We have two validations for user age: One to see if it's present, another to see if it's in the list of accepted age values.

code-dot-org/dashboard/app/models/user.rb

Lines 407 to 412 in 84bba2a

	defer_age = proc {|user| user.provider == 'google_oauth2' || user.provider == 'clever'}
	validates :age, presence: true, on: :create, unless: defer_age # only do this on create to avoid problems with existing users
	AGE_DROPDOWN_OPTIONS = (4..20).to_a << "21+"
	validates :age, presence: false, inclusion: {in: AGE_DROPDOWN_OPTIONS}, allow_blank: true

In #15882 @joshlory correctly set us up to bypass the first age validation for Google Classroom since a birthdate is not guaranteed to be provided via OAuth. In #16632 he did the same for Clever. The second validation still runs, but when an age isn't provided it passes as well.

However, there is an edge case we didn't account for: If a birthdate is provided, but the user is less than four years old, we skip the first validation and fail the second. I verified this with a new set of user model tests in this PR:

I think a reasonable behavior in this case is to act as if a birthdate wasn't provided at all, so the user will be prompted to provide an age when they log in.

[joshlory]

LGTM. Thanks for fixing!