Fix CodeQL Analyze (c-cpp) autobuild failure by removing std::generator dependency - #208

The Analyze (c-cpp) GitHub Actions job was failing during CodeQL autobuild because lib/xml/xmlparser.cpp included <generator>, which is unavailable in the runner toolchain. This blocked compilation of the XML parser translation unit and aborted analysis.

• Root cause

  • xmlparser.cpp depended on C++23 coroutine generator support (std::generator), but CI’s compiler/libstdc++ combination does not provide <generator>.

• Code changes

  • Replaced coroutine-based iterators with callback-based iteration in lib/xml/xmlparser.cpp.
  • Removed #include <generator>.
  • Converted:
    • iterateElementChildren(...) from std::generator<DOMElement*> to callback traversal.
    • splitMessages(...) from std::generator<MessageSlice> to callback traversal with bool-based early exit.
  • Added a compile-time contract (static_assert) requiring iterateElementChildren callbacks to return void to prevent ambiguous callback semantics.

• Behavioral intent

  • Preserve existing parse/framing behavior while removing a non-portable standard library dependency that breaks CodeQL autobuild.

// Before
std::generator<MessageSlice> splitMessages(std::string_view input, bool& framingError);

// After
template <typename Callback>
void splitMessages(std::string_view input, bool& framingError, Callback&& handler) {
    // handler(slice) -> true to continue, false to stop
}