mini-string

My implementation of core string functions with basic error handling and test coverage. Recreates essential string operations from the C standard library.

String Functions:

• s21_strlen() - calculates string length with NULL safety
• s21_strcmp() - compares strings with NULL handling
• s21_strcpy() - safe string copying with buffer validation
• s21_strcat() - concatenates strings with input checking
• s21_strchr() - locates first character occurrence

Memory Functions:

• s21_memcpy() - copies memory blocks with overlap protection
• s21_memset() - fills memory with constant byte
• s21_strstr() - finds substring with boundary checks

Testing:

The project includes test cases covering:

• Normal operation scenarios
• Edge cases (NULL pointers, empty strings)
• Buffer boundary conditions
• Character search functionality