cast to unsigned char before std::isprint in jsonParseStrict

[metsw24-max]

Signed char passed to std::isprint in jsonParseStrict

The formatter walks the nlohmann error string a char at a time and hands each value to std::isprint. Because the message echoes the offending input bytes, anything from 0x80 upward arrives as a negative int on signed-char builds, which falls outside the range the <cctype> overload is defined for. Casting to unsigned char first keeps the argument valid and lines up with the isPrint helper already used elsewhere in the tree.

The path is reachable from untrusted input via the standard-JSON interface and the LSP transport, so it seemed worth tidying even though most libc builds happen to tolerate it.