Site Address accepts emails when trying to log in using an URL

I've seen a few cases when users are getting an incorrect username/password message but they are sure that the credentials are correct. What is probably happening (example on 3729175-zen) is that users type their own email instead of their site URL. And for some reason, we accept that if the part after the @ is a WP site.

Expected behavior

To see an error message explaining to the user that this field is for site URLs, not emails.

Actual behavior

1. If you type an email (valid or not) followed by a domain that is not pointing to a WP site, you'll see the 'The site at this address is not a WordPress site` message.
2. If you type an email (valid or not) followed by a domain that IS pointing to a WP site, we will let the user proceed and ask them to log in (to an email).

Image from https://github.com/wordpress-mobile/WordPress-iOS/issues/15868 :

Steps to reproduce the behavior

1. Open the app while unlogged
2. Click on Enter your existing site address
3. Type something like http://lekrkr@example.com. Make sure that example.com is a WP site.

Tested on iPhone 7, iOS 14.4, WPiOS 16.5 and 16.6

Reproduce the issue from the “Enter your existing site address” flow using an email-like address whose domain is a WordPress site, then trace the site-address validation and login entry point. Done means email-like input shows an error explaining that the field accepts site URLs, while valid site URLs still proceed normally.