[SYCL] Add noexcept and hidden friends to nd_item (#22902)
Fixes #22879.
This PR aligns `sycl::nd_item` with the SYCL 2020 specification:
- Adds `noexcept` to the standard member functions.
- Implements `operator==` and `operator!=` as hidden friends.
- Adds compile-time interface tests covering exception specifications
and comparison operators.
