feat(br): support new cnpj format

[taj]

Hi!

Brazil's CNPJ format is expanding to allow letters A-Z in the first 12 positions , while the last 2 check digits remain numeric.

Backward-compatible — existing numeric CNPJs validate identically.

Reference: https://kpmg.com/us/en/taxnewsflash/news/2026/03/tnf-brazi-new-alphanumeric-format-for-taxpayer-registry.html

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This is my first contribution to this project, so happy to adjust the approach if it doesn't align with how you'd prefer to handle this. Let me know if any changes are needed!

And thank you!

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces crucial updates to the Brazilian CNPJ validation and formatting utilities. It enables the system to correctly process the upcoming alphanumeric CNPJ format, which will be adopted starting July 2026, while ensuring that all existing numeric CNPJs continue to be handled without issue. The changes primarily involve adapting the character interpretation and validation logic to accommodate letters in the identifier's main body.

Highlights

• Alphanumeric CNPJ Support: Implemented support for the new alphanumeric Brazilian CNPJ format, which allows letters A-Z in the first 12 positions while retaining numeric check digits.
• Backward Compatibility: Ensured that existing numeric CNPJs remain valid and are processed identically, maintaining backward compatibility.
• Checksum Calculation Update: Modified the checksum calculation logic to correctly interpret alphanumeric characters based on their ASCII values for the first 12 positions.
• Enhanced Validation: Updated validation rules to accept alphanumeric characters in the main body of the CNPJ and specifically enforce numeric characters for the two check digits.
• Comprehensive Test Cases: Added new test cases to cover formatting and validation of alphanumeric CNPJs, including scenarios with lowercase input, incorrect checksums, and invalid check digits.

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[gemini-code-assist]

Code Review

This pull request updates the CNPJ validation logic to support the new alphanumeric format starting in July 2026, while maintaining compatibility with legacy numeric-only CNPJs. The implementation correctly updates the checksum calculation and validation rules. I have provided a high-severity feedback comment suggesting that the clean function should normalize inputs to uppercase to ensure consistent checksum calculation and formatting for alphanumeric CNPJs.