Added support for validating private fields

[alexaandru]

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[Copilot]

Pull Request Overview

This pull request adds support for validating private (unexported) struct fields by introducing a new Interface function that works around Go's reflection limitations. The key change removes the ErrorOnPrivate flag and the associated ErrPrivateField error, allowing private fields with validation tags to be validated automatically.

• Adds Interface helper function to extract values from private fields using reflection
• Removes ErrorOnPrivate configuration option and ErrPrivateField error
• Updates all validation checkers to use Interface instead of direct reflect.Value.Interface() calls
• Adds comprehensive test coverage for private field validation

Reviewed Changes

Copilot reviewed 9 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
vali.go	Removes ErrorOnPrivate field, adds Interface function for private field access, removes private field error handling in validation logic
check.go	Updates all checker functions to use Interface helper, removes ErrPrivateField error, improves type handling in luhn and sizeCmp functions
vali_test.go	Adds extensive test cases for private field validation, adds TestInterface function to verify the Interface helper
example_test.go	Updates example to demonstrate private field validation instead of the removed ErrorOnPrivate feature
README.md	Updates documentation to reflect support for private field validation, removes ErrorOnPrivate examples
go.mod, tools/go.mod, tools/go.sum	Updates Go version to 1.25.4 and updates various tool dependencies
coverage-badge.svg	Adds coverage badge showing 97.4% test coverage
Makefile	Updates formatting commands and adds coverage badge generation
.golangci.yml	Configuration updates including line length increase and linter adjustments

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