Add Credit Card Number Validation Functionality

[vitochillsme-boop]

Implement a function that validates credit card numbers using the Luhn algorithm.

Requirements:

• Accepts a credit card number as input (string or integer).
• Validates the number using the Luhn algorithm.
• Returns a boolean indicating validity.

Background:
Credit card validation is a common requirement for payment processing and user input verification. The Luhn algorithm is the industry standard for verifying the structure of card numbers before further processing.

Acceptance Criteria:

• The validation function is added to the codebase (e.g., cc_validation.py).
• Include basic unit tests demonstrating the function works with valid and invalid card numbers.

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