Fix constexpr std::string compatibility in deps/ada/ada.h

Older libstdc++ (GCC < 12) doesn't support constexpr std::string.
Add ADA_CONSTEXPR macro that falls back to non-constexpr on older
toolchains while preserving constexpr on newer ones.

Co-Authored-By: Claude (global.anthropic.claude-opus-4-5-20251101-v1:0) <noreply@anthropic.com>

Author: JackStrohm-asana