[GH-2798] Add ST_ShortestLine function

[jiayuasu]

Summary

• Add ST_ShortestLine(geom1, geom2) function that returns the shortest LineString connecting two geometries
• Uses JTS DistanceOp.nearestPoints() internally — same approach as the existing ST_ClosestPoint but returns both endpoints as a LineString instead of a single Point
• Returns null for empty geometries

Closes #2798

Changes

Implementation (6 layers)

• Java core (Functions.java): shortestLine(Geometry, Geometry) using DistanceOp
• Scala Expression (Functions.scala): ST_ShortestLine case class
• Catalog (Catalog.scala): Function registration
• Scala API (st_functions.scala): Column/String wrappers
• Python API (st_functions.py): Python wrapper
• Flink (Functions.java, Catalog.java): Flink scalar function + registration

Tests

• Java unit tests (5 cases: point-to-point, point-to-linestring, polygon-to-polygon, empty geometry, same geometry)
• Scala SQL tests + DataFrame API test
• SRID preservation test
• Python SQL test + DataFrame API test
• Flink test

Documentation

• SQL, Flink, and Snowflake docs with examples

Test plan

• Java unit tests pass (mvn test -pl common -Dtest="FunctionsTest#shortestLine*")
• Python SQL tests pass
• Python DataFrame API tests pass
• CI full test suite