Python bindings: support __floordiv__ for index types

    The original reimplementation of EDSC as declarative builders and the
    subsequent rework of Python bindings forbade to use the (true) division
    operator for values of the index types without providing an alternative.  Index
    types only support floor and ceil division through affine maps.  Expose this to
    Python bindings through a `__floordiv__` function on `ValueHandle`s.

--

PiperOrigin-RevId: 241713093