Create calculator with add and multiply functions

[mengotdibran11-cyber]

Implement a calculator in C that can perform basic arithmetic functions: addition and multiplication. The calculator should accept two numbers as input and return the result for both operations.

Requirements:

• Addition function: Takes two numbers, returns their sum.
• Multiplication function: Takes two numbers, returns their product.
• Input validation: Ensure only valid numbers are processed.
• Provide example usage in code comments/documentation.

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[mengotdibran11-cyber]

#1875