geotiff: assert spec.dtype in parity matrix harness (#1985)

Second review-pr pass on PR #2005 caught that ``_FixtureSpec.dtype``
was declared and documented but never read by ``assert_parity``. The
only dtype check was reference-vs-actual inside ``_assert_pixels_equal``,
which means a backend bug that uniformly upcasts in both the reference
read and the backend read would not be caught.

Assert ``actual.dtype == spec.dtype`` against the spec so a silent
upcast that the reference also exhibits still fails the cell. Update
the ``assert_parity`` docstring to match what the function actually
does (dims/coords/transform vs reference; dtype, crs, nodata vs spec).

Author: brendancol

xrspatial/geotiff/tests/test_backend_parity_matrix.py

@@ -256,15 +256,23 @@ def assert_parity(
     on-disk fixture, used to build the eager-numpy reference.
 
     The eager-numpy read of the same file is the reference for the pixel
-    array, coord values, dtype, dims, and transform tuple. Attrs are
-    asserted against the spec rather than against the reference, so a
-    bug that drops an attr in *every* backend still fails this cell.
+    array, coord values, dims, and transform tuple. ``spec.dtype`` and
+    ``spec.expected_crs_epsg`` / ``spec.expected_nodata`` are asserted
+    against the actual independently of the reference, so a bug that
+    silently changes them in *every* backend still fails this cell.
     """
     ref = open_geotiff(str(path))
 
     # Pixel array, dtype, shape.
+    actual_arr = _materialise(da)
     _assert_pixels_equal(
-        _materialise(ref), _materialise(da), label=label,
+        _materialise(ref), actual_arr, label=label,
+    )
+
+    # Dtype against the spec, not just against the reference. Catches a
+    # silent upcast that the reference would also exhibit.
+    assert actual_arr.dtype == spec.dtype, (
+        f"{label}: dtype {actual_arr.dtype} != spec dtype {spec.dtype}"
     )
 
     # Dims + order.