char* cpy (char* destination, const char* source)

[Description]: The copy function shall copy one string from source include '\0' to destination and return the copied string. Only can happen if the size of the destination is large enough to store the copied string, else returns NULL.

*****************without using built-in C++ string so don't use strlen() or anything from #include *****************

Also, write some test cases in functions with the following name style: [function]length__test_case_[n] where n= test case number

Have enough test cases to show the code properly works.

Please don't use ai generated answers.

Thank you.