fix double-free in CZString copy-assignment operator

[SABITHSAHEB]

CZString is the key type behind Json::Value's object and array maps, and a key that owns its buffer (the duplicate policy) frees that buffer in its destructor. The copy-assignment operator only shallow-copied cstr_ and the packed index_/policy word, so after a = b both keys pointed at b's allocation while a's old buffer was never released. On scope exit both destructors free the same pointer, which AddressSanitizer reports as a double free, and the overwritten buffer leaks. I ran into it while filling out the CZString assignment coverage from #1654, which exercised the copy and move constructors and the move-assignment but not this operator. The move-assignment already releases then transfers correctly, so I made copy-assignment do the safe thing with copy-and-swap: the temporary deep-copies the source and takes our old buffer to be freed exactly once. Added the missing case to runCZStringTests, which double-frees under ASan before the change and passes after.