[KartikeyG-world]

Summary

• Use the standard lld integer format for MinGW-w64 builds targeting UCRT while preserving the existing format for other Windows configurations.
• Apply corresponding UCRT detection during CMake configuration.
• Add a regression test for exact serialization of a large 64-bit integer.

Closes #752

Validation

• git diff --check passed.
• A C11 syntax-only compilation was attempted but could not proceed because the generated jansson_private_config.h file was unavailable.
• The full project test suite was not run.
• UCRT and legacy MSVCRT behavior have not been verified locally because the required toolchains are unavailable.