Refactor backup code verification to follow conventional form pattern

[aduth]

🎫 Ticket

Supports LG-14078

🛠 Summary of changes

Updates backup code verification to use the conventional form pattern:

• Validation should use ActiveModel#valid? in combination with validation methods appending errors as relevant
• Include errors in FormResponse return value
• Use errors from form response result to display in user interface
• Remove unused BackupCodeGenerator#verify method and update specs to ensure coverage for actively-used methods

To simplify review, consider hiding whitespace changes: https://github.com/18F/identity-idp/pull/11089/files?w=1

📜 Testing Plan

Verify that you can sign in with backup codes:

1. Prerequisite: Have an account with backup codes where you have a record of a usable code
2. Go to http://localhost:3000
3. SIgn in
4. If not prompted for MFA, click "Forget all browsers" from account dashboard, confirm, sign out, then start from Step 3
5. When prompted for MFA, if prompted for an MFA other than backup codes, select "Choose another authentication method" and select "Backup codes"
6. When prompted for backup codes, enter a valid backup code and submit
7. Observe no regressions in being signed in

Verify that updated specs pass:

rspec spec/controllers/two_factor_authentication/backup_code_verification_controller_spec.rb spec/forms/backup_code_verification_form_spec.rb spec/services/backup_code_generator_spec.rb

[kevinsmaster5]

Looks good. Local testing OK - no errors.