end() is allowed to return a different type (a sentinel), so operator== knows when it is doing an end check, and can do >= like BSONObj, rather than ==. This ensures that if we somehow end up past the end we exit the loop rather than continuing forever. Alternatively, we could throw in the > case, since something is very wrong if we get there.
operator* doesn't need to return a `T&`, so we can return a BSONElement by value rather than storing one as a member. This gives us more flexibility in how we represent the data in the iterator.
Consider implementing a BSONObj std::ranges::view_interface
Improvement
Major - P3