refactor(geospatial): remove StructuredGrid support, ensure consistent return types

- Remove StructuredGrid-specific code from GeospatialIndex (uses its own
  optimized intersect method)
- Fix StructuredGrid.intersect to return (nan, nan, nan) instead of
  (None, nan, nan) for outside points with z coordinate
- Update GeospatialIndex.query_point to return np.nan instead of None
  for outside points, matching native methods
- Update GeospatialIndex.query_points to return int64 array when all
  points are inside, float64 only when nan values present
- Add comprehensive tests for return type consistency across all grid
  types (StructuredGrid, VertexGrid, UnstructuredGrid)
- Add tests for 1:1 point-to-cell mapping behavior
- Add tests for 3D layered UnstructuredGrid tie-breaking

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Co-Authored-By: Claude <[email protected]>

Author: adacovsk