slope: widen name type hint to Optional[str] for terrain-family parity

[brendancol]

Closes #2681

slope() annotated its name parameter as str, but aspect(), curvature(), northness(), and eastness() all use Optional[str]. Since name flows into xr.DataArray(name=name) (which accepts None), slope(agg, name=None) already worked at runtime; the annotation was just wrong.

• Widen name: str = 'slope' to name: Optional[str] = 'slope' and import Optional.
• Add a test asserting the name annotation matches the rest of the terrain family.
• Add a test that slope(agg, name=None) returns a DataArray with name is None.

Widening a type hint is non-breaking, so no deprecation shim is needed.

Backend coverage: type-hint-only change on the public entry point; no backend logic touched. numpy / cupy / dask+numpy / dask+cupy paths are unchanged.

Test plan:

• pytest xrspatial/tests/test_slope.py (43 passed locally)
• new parity + name=None tests pass

[brendancol]

Domain-aware review (api-consistency sweep, slope module).

Scope: widen slope()'s name annotation from str to Optional[str] so it matches aspect, curvature, northness, and eastness, plus two tests.

Blockers: none.

Suggestions: none. The change is the minimal correct fix. name flows straight into xr.DataArray(name=name), which accepts None, so widening the hint matches runtime behavior and the rest of the terrain family. No deprecation shim is needed since widening a type hint is non-breaking.

Nits: none. Optional import is placed correctly alongside the existing Union import; both are now used.

Verification:

• get_type_hints(slope)['name'] resolves to str | None, matching aspect.
• slope(agg, name=None) returns a DataArray with name is None.
• Full test_slope.py passes (43 tests). No backend logic touched, so numpy/cupy/dask paths are unaffected.