Fix: Use py::ssize_t instead of ssize_t in span tests

On Windows/MSVC, ssize_t is not available in the standard namespace
without proper includes. Use py::ssize_t (the pybind11 typedef) instead
to ensure cross-platform compatibility.

Fixes compilation errors on:
- Windows/MSVC 2022 (C++20)
- GCC 10 (C++20)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: rwgk

tests/test_numpy_array.cpp

@@ -251,11 +251,11 @@ TEST_SUBMODULE(numpy_array, sm) {
     // test_shape_strides_span
     sm.def("shape_span", [](const arr &a) {
         auto span = a.shape_span();
-        return std::vector<ssize_t>(span.begin(), span.end());
+        return std::vector<py::ssize_t>(span.begin(), span.end());
     });
     sm.def("strides_span", [](const arr &a) {
         auto span = a.strides_span();
-        return std::vector<ssize_t>(span.begin(), span.end());
+        return std::vector<py::ssize_t>(span.begin(), span.end());
     });
     // Test that spans can be used to construct new arrays
     sm.def("array_from_spans", [](const arr &a) {