Fix reCAPTCHA add phone fallback links

[aduth]

🛠 Summary of changes

Fixes an issue where the reCAPTCHA fallback screen shows incorrect links when adding a phone number to an existing account.

This was an unintended side-effect of the phone consolidation in #8966, since the links were controlled by distinct logic in the two controllers, where one would provide locals value to the view to render a link to "Choose another authentication method":

identity-idp/app/controllers/users/phones_controller.rb

Line 27 in c068dfb

render 'users/phone_setup/spam_protection'

identity-idp/app/controllers/users/phone_setup_controller.rb

Line 38 in c068dfb

render :spam_protection, locals: { authentication_methods_setup_path: }

There was not sufficient regression coverage for this behavior, hence why it was not caught previously.

📜 Testing Plan

1. Go to http://localhost:3000
2. Create an account
3. Select phone as MFA during setup
4. At phone setup screen, use phone number +610491570006 and assign a score 0.4
5. Click "Send code"
6. Observe in fallback screen the "Choose another authentication method" link. Optionally click it to return to MFA selection screen.
7. Continue account creation.
8. Once account is created, click "Add phone number" from account dashboard
9. At phone setup screen, use phone number +610491570156 and assign a score 0.4
10. Click "Send code"
11. Observe no "Choose another authentication method" link. Instead, see link to "Cancel" to return to account dashboard.

👀 Screenshots

Screen	Before	After
New Account Fallback
Existing Account Fallback