[GH-2884] Add ST_Extent aggregate (returns Box2D)

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• Yes, and the PR name follows the format [GH-XXX] my subject. Closes Implement ST_Extent aggregate (returns Box2D) #2884

What changes were proposed in this PR?

Adds the ST_Extent(geom) aggregate, returning a Box2D (the union of all bounding boxes in the column). PostGIS-compatible.

SELECT ST_Extent(geom) FROM my_table;
-- BOX(xmin ymin, xmax ymax)

Reuses the existing EnvelopeBuffer reducer/merger from ST_Envelope_Aggr; only the finish step differs — Box2D instead of a polygon Geometry. ST_Envelope_Aggr is left untouched for backwards compatibility.

Behavior:

• Empty input (zero rows) → NULL.
• All rows null or empty geometry → NULL.
• Mixed null/empty/valid rows → bbox of the valid rows only; null and empty rows are ignored.

How was this patch tested?

aggregateFunctionTestScala:

• "Passed ST_Extent" — three geometries (two points + one linestring) reduce to the expected box.
• "ST_Extent returns null over zero rows" — empty input filtered to zero rows → NULL.
• "ST_Extent returns null when all inputs are null or empty" — all-null/empty input → NULL.
• "ST_Extent ignores null and empty rows mixed with valid geometries" — mixed input reflects valid geometries only.

Did this PR include necessary documentation updates?

• No, this PR does not affect any public SQL API documentation surface in isolation. Documentation for ST_Extent lands with the rest of the Phase 1 surface (Add a native Box2D type for bounding boxes #2877) once the cast/text functions exist. Scala DataFrame API wrappers tracked separately in Scala/Python DataFrame API wrappers for Box2D Phase 1 functions #2891.