Python bindings for Box2D scalar functions and ST_Extent

[jiayuasu]

Sub-task of #2877.

Scope

Mirror the Phase 1 SQL surface in PySpark wrappers:

• ST_Box2D(geom) in python/sedona/spark/sql/st_functions.py
• ST_MakeBox2D(p1, p2) in python/sedona/spark/sql/st_functions.py
• ST_Extent(geom) in python/sedona/spark/sql/st_aggregates.py
• Accessor overloads (ST_XMin/XMax/YMin/YMax(box)) — these wrappers already exist for Geometry; verify they pass Box2D arguments through cleanly.
• ST_AsText(box) — same as above.

The Python Box2DType UDT and Box2D value class are already merged in #2878.

Implementation

• Each function uses the existing _call_st_function helper to dispatch to the JVM-side function.
• Imports of Box2DType / Box2D for users who want to construct or assert on bbox columns.
• Python tests: each function on small DataFrames, NULL propagation, schema check that bbox columns surface as Box2DType.

Depends on

• Implement ST_Box2D(geom) scalar function #2881 (ST_Box2D)
• Overload ST_XMin/XMax/YMin/YMax for Box2D #2882 (accessor overloads)
• Implement ST_MakeBox2D(p1, p2) scalar constructor #2883 (ST_MakeBox2D)
• Implement ST_Extent aggregate (returns Box2D) #2884 (ST_Extent)
• Add CAST(box2d AS geometry) and ST_AsText(box2d) #2885 (ST_AsText)