Escape msg before embedding in JSON payload in mps_parser_throw

Summary

mps_parser_throw() (in cpp/src/io/utilities/error.hpp) concatenates msg verbatim into a JSON string. A token, file path, or system error message containing ", \, or a newline will produce malformed JSON and break any downstream consumer expecting the MPS_PARSER_ERROR_TYPE / msg structure.

Suggested fix

Add a small json_escape(const std::string&) helper that replaces \, ", \n, \r, and \t with their JSON-escaped forms, and apply it when constructing the thrown std::logic_error message in mps_parser_throw.

References

• Raised during review of PR #1429 (experimental fast MPS parser): https://github.com/NVIDIA/cuopt/pull/1429#discussion_r3404570407
• Deferred from that PR by @aliceb-nv to be addressed in a follow-up.