Fix deprecated unary_union in mergeinstances

[wanjing-1116]

This PR updates the cross-tile merge logic to accommodate recent changes
in Shapely’s geometry API.

In newer versions of Shapely, the unary_union attribute is deprecated,
and the recommended replacement is geometry.union_all(). In addition,
the union operation may return either a Polygon or a MultiPolygon,
whereas the previous implementation implicitly assumed a MultiPolygon
and accessed .geoms unconditionally.

This PR replaces the deprecated unary_union call with
geometry.union_all() and adds explicit handling for Polygon vs
MultiPolygon outputs. This prevents tile-level merge failures that
occurred due to the changed return type behavior, and restores stable
cross-tile merging without altering the original merge semantics.

Tested on the Nigeria 2024 configuration: the merge completes
successfully and the merged GeoParquet is generated as expected.