Set slope output units to 'degrees' instead of leaking input units

[brendancol]

Closes #2894

slope() returns degrees but copied the input DataArray's attrs verbatim, so a DEM with units='m' produced a slope array still labelled units='m'. _infer_vertical_unit_type reads that label and treated the slope output as elevation rather than an angle.

Changes

• Override attrs['units'] to 'degrees' in the shared public wrapper, so the fix applies to all backends and to Dataset inputs. All other input attrs carry through unchanged, and the input array is not mutated.
• Update the docstring Returns note to state the units override.
• Add a verify_attrs passthrough to the cross-backend equality helpers in general_checks.py; the slope tests whose attrs now legitimately differ pass verify_attrs=False.
• Add tests asserting the degrees override on numpy, cupy, dask+numpy, dask+cupy, the no-input-units case, and the geodesic path.

Backend coverage

numpy, cupy, dask+numpy, dask+cupy (attrs are set once in the wrapper).

Test plan

• pytest xrspatial/tests/test_slope.py xrspatial/tests/test_geodesic_slope.py (121 passed)
• Regression check on aspect/curvature/hillshade/terrain_metrics/northness_eastness which share the helpers (565 passed)

[brendancol]

PR Review: Set slope output units to 'degrees' instead of leaking input units

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

• xrspatial/tests/test_slope.py: flipping the existing tests to verify_attrs=False means they no longer confirm that the other input attrs (res, crs) survive slope(). Only test_units_overridden_to_degrees_numpy checks res/crs propagation, and only on numpy. Worth having that numpy test assert the non-units attrs explicitly so the propagation contract stays covered now that the blanket attrs check is off.

Nits (optional improvements)

• None.

What looks good

• The override lives in the single shared wrapper, so all four backends and the Dataset path inherit it without per-backend edits. numpy/cupy/dask+numpy/dask+cupy and Dataset all report 'degrees'.
• dict(agg.attrs) copies before mutating, so the input DataArray's attrs aren't touched; a test asserts the input still has units='m'.
• The geodesic path is covered.
• The verify_attrs passthrough on the shared equality helpers defaults to True, so no other module's tests change behavior.

Checklist

• slope is degrees, units now say degrees
• All implemented backends produce consistent results
• NaN handling unaffected by this change
• Edge cases covered (no-input-units case, geodesic path)
• Dask chunk boundaries unaffected
• No premature materialization or extra copies (one dict copy of attrs)
• Benchmark not needed
• README feature matrix not applicable (no new function)
• Docstring updated to note the units override

[brendancol]

Follow-up review

The one suggestion from the previous pass is addressed: test_units_overridden_to_degrees_dask_numpy now asserts res and crs survive slope() on the dask path, in addition to the numpy test. No blockers, suggestions, or nits remain.