Feat: implement email tag validation

[Copilot]

This PR implements email validation support for the validgen code generator, enabling developers to validate email fields in Go structs using the email tag.

Features Added

• Email validation tag: Added support for email validation tag that validates email format using regex
• Optional field support: Empty strings are considered valid for optional email fields
• Required + email combination: Works seamlessly with required tag for mandatory email fields

Usage Examples

type User struct {
    Email1 string `validate:"required,email"`  // Required and must be valid email
    Email2 string `validate:"email"`           // Optional, but if present must be valid email
}

Implementation Details

• Added IsValidEmail() function in types/string_utils.go with comprehensive regex-based validation
• Extended parser to recognize "email" as a zero-value validation tag
• Added email validation logic to the code generation condition table
• Comprehensive test coverage for various email formats (valid/invalid cases)

Validation Examples

// These will pass validation
user1 := &User{Email1: "user@domain.com", Email2: ""}                    // Valid required, empty optional
user2 := &User{Email1: "test@example.org", Email2: "optional@test.co.uk"} // Both valid

// These will fail validation
user3 := &User{Email1: "", Email2: ""}                 // Required email is empty
user4 := &User{Email1: "invalid.email", Email2: ""}    // Invalid email format
user5 := &User{Email1: "user@domain.com", Email2: "bad"} // Invalid optional email

The generated validation code efficiently checks email format without reflection, maintaining validgen's performance benefits.

Fixes #5.

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[alexgarzao]

Very nice contribution! Thanks @leocavalcante !

[leocavalcante]

@copilot create an end-to-end test for the email validation at the tests/endtoend/string.go file.

[fabriciogpp]

A propósito https://www.youtube.com/watch?v=xxX81WmXjPg 😅

[fabriciogpp]

O problema aqui é que vai compilar em cada chamada de validação.

[leocavalcante]

@copilot how can we avoid the regex from being compiled at each request? Can we have a pre-compiled regex to be reused for subsequent email validations?

[Copilot]

@copilot create an end-to-end test for the email validation at the tests/endtoend/string.go file.

I've created comprehensive end-to-end tests for email validation in tests/endtoend/string.go. The tests cover:

• Required email field validation (empty and invalid format errors)
• Optional email field validation (empty valid, invalid format errors)
• Valid email formats for both required and optional fields
• Integration with existing string validation tests

All tests are passing and verify the generated validation code correctly handles email validation scenarios. Commit: a564fac

[leocavalcante]

@copilot how can we avoid the regex from being compiled at each request? Can we have a pre-compiled regex to be reused for subsequent email validations?

[Copilot]

@copilot how can we avoid the regex from being compiled at each request? Can we have a pre-compiled regex to be reused for subsequent email validations?

I've optimized the email validation by using a pre-compiled regex. Instead of compiling the regex on each validation call, the regex is now compiled once at package initialization and reused for all subsequent validations. This provides better performance, especially when validating many emails. The function signature and behavior remain exactly the same. Commit: ed4e6fa