fix: enable reliable cross-platform test execution

[R3na7]

Summary

• Enable CTest test discovery in the root CMake configuration
• Make CI fail when no tests are discovered
• Run the test suite through CTest in the README instructions
• Make the noexcept interface test portable across compilers and standard libraries

Validation

• CTest discovers the GoogleTest suite
• Tests run on GCC, Clang, MSVC, and AppleClang
• ASan and UBSan checks remain enabled
• clang-format validation remains enabled