Should WString use memcpy not strcpy

In WString.cpp (this also applies to avr core)

https://github.com/arduino/ArduinoCore-samd/blob/master/cores/arduino/WString.cpp#L179 and
https://github.com/arduino/ArduinoCore-samd/blob/master/cores/arduino/WString.cpp#L271

strcpy is used, where are length is passed to the copy and concat operation, whereas memcpy should be used so the char* can contain the 0 character.

Open WString.cpp and inspect the two referenced locations, lines 179 and 271, along with the corresponding AVR core implementation mentioned in the issue. Verify the copy and concatenation behavior when the source char* contains an embedded zero, and confirm both SAMD and AVR paths handle the provided length correctly.

Written by the indexing model from the issue text.