Refactor Password Variables to Use `const char[]` Arrays

- Converted password declaration from `char*` to `const char[]`.
- Updated `memcpy` and `return` statements accordingly to use `sizeof` instead of predefined lengths.
- Renamed `key_password` into `weak_password` to match test name.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25330)

(cherry picked from commit d52e92f8)