Fix email validation to reject TLD-only domains like user@com

[Lucas-BRT]

This pull request enhances the email validation logic in validator/src/validation/email.rs to ensure stricter compliance with domain validation rules. It also updates the corresponding test cases to reflect these changes and adds new tests to cover edge cases.

Email Validation Logic Updates:

• Modified validate_domain_part to reject email addresses with top-level domains (e.g., user@com) by requiring at least one dot in the domain part. This ensures stricter validation of domain structures.

Test Case Updates:

• Updated an existing test case to mark abc@bar as invalid, aligning with the stricter domain validation rules.
• Added a new test case test_user_at_com_fails to validate that user@com is correctly identified as an invalid email address.
• Introduced a test in validator_derive_tests/tests/email.rs to ensure that custom implementations for email validation also reject top-level domain-only email addresses. This includes detailed error assertions to verify the validation logic.

[Keats]

Technically user@com can be a valid email. The current regex matches the HTML5 email regex that does allow it

[Lucas-BRT]

Although HTML5 allows email addresses like user@com with its internal regex, this does not reflect the stricter rules required by actual email validation standards.

HTML5 validation, as shown in W3Schools, does not allow user@com.

Allowing this could lead to issues with DNS systems and other technical risks, as outlined in ICANN's guide on dotless domains.

For more robust validation, stricter rules should be applied.

RFC 5321: https://datatracker.ietf.org/doc/html/rfc5321

RFC 5322: https://datatracker.ietf.org/doc/html/rfc5322

W3Schools: https://www.w3schools.com/tags/att_input_pattern.asp

ICANN Guide: https://www.icann.org/en/announcements/details/new-gtld-dotless-domain-names-prohibited-30-8-2013-en

[pvichivanives]

thoughts on just making this another type like email_strict or email_domain