Box3D aggregate: ST_3DExtent

[jiayuasu]

Follow-up to the Box3D Phase 1 epic (#2973). Adds the Box3D aggregate.

Scope

• ST_3DExtent(geom) -> box3d — aggregate that returns the planar 3D bounding box over a column of Geometry. Mirrors PostGIS's ST_3DExtent and parallels Sedona's existing ST_Extent (which returns a Box2D).

Semantics

• Geometries without a Z dimension contribute z = 0, matching ST_Box3D's PostGIS-compatible default.
• NULL inputs are skipped.
• Empty input set → NULL.
• Result is a Box3D (struct of six non-nullable doubles in PostGIS storage order: xmin, ymin, zmin, xmax, ymax, zmax).

Out of scope

• Python / Flink / R bindings for the aggregate.
• Documentation (will land alongside the rest of the Phase 1 docs PR, analogous to Box2D's [DOCS] Add Box2D SQL documentation #2966).