Add compile-time tests for mdspan-style View template arguments with helper abstraction

[Copilot]

Adds tests validating that Kokkos::View constructed with mdspan-style template arguments (scalar, extents, layout, accessor) produces identical mdspan_type as traditional Kokkos style (datatype, layout, space, memorytraits).

Changes

• New test file: core/unit_test/view/TestViewMDSpanStyleTemplateArgs.cpp

  • 15 compile-time assertions covering 1D-6D arrays, LayoutLeft/Right, memory traits (Unmanaged, Atomic, RandomAccess, Restrict, combinations), const types, and different execution spaces
  • Uses Kokkos::Experimental::Accessor<ElementType, Space, MemoryTraits> to construct accessors

• Helper abstraction (added in separate commit):

  • test_mdspan_view_equivalence<ElementType, Rank, Layout, Space, MemTraits>: constructs both View styles and compares their mdspan_type
  • rank_to_asterisks<Rank>: maps rank integer to pointer syntax (T*, T**, etc.)
  • layout_to_mdspan<Layout>: maps LayoutLeft/Right to layout_left/right_padded<dynamic_extent>

Example

Traditional verbose assertion (10 lines):

static_assert(
    std::is_same_v<
        Kokkos::View<double*, Kokkos::DefaultExecutionSpace>::mdspan_type,
        Kokkos::View<double, Kokkos::dextents<size_t, 1>,
                     Kokkos::Experimental::layout_right_padded<Kokkos::dynamic_extent>,
                     Kokkos::Experimental::Accessor<double, Kokkos::DefaultExecutionSpace,
                                                    Kokkos::MemoryTraits<>>>::mdspan_type>);

Refactored with helper (3 lines):

static_assert(
    test_mdspan_view_equivalence<double, 1, Kokkos::LayoutRight,
                                 Kokkos::DefaultExecutionSpace>::value);

File reduced from 197→174 lines (12% reduction) while maintaining identical test coverage.

Original prompt

We need to add tests for the use of mdspan style template arguments in Kokkos::View. I.e. using scalartype, extents, layout and accessor as template arguments instead of datatype, layout, space, and memorytraits. The initial test should be a compile only tests asserting that the mdspan_type of equivalent Views defined in the two styles is the same.

For this test leverage accessors defined via Kokkos::Experimental::Accessor<scalar_type, space, MemoryTraits<SomeTraits>> only. For quick compilation tests while you work on this, add the test initially to the core/unit_test/default/TestDefaultDeviceDevelop.cpp which can be build by building the target Kokkos_CoreUnitTest_Develop. Only move it to a new file in the core/unit_test/view directory when you are happy with it.

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