Fix Polygon cast returning bare geometry instead of SpatialExpression

[halkibsi]

Summary

Casts\Polygon::set() returned the bare Polygon object for typed geometries, while Point and MultiPolygon wrap theirs in SpatialExpression.

Because Laravel re-runs class casts during getAttributes() (mergeAttributesFromClassCasts), the second set() call overwrote the SpatialExpression that SpatialTrait::performInsert() had prepared — so the raw Polygon object was bound to the query, PDO stringified it to (lng lat, ...), and MySQL rejected the write:

SQLSTATE[22003]: Numeric value out of range: 1416 Cannot get geometry
object from data you send to the GEOMETRY field

Hit this running the FleetOps testing seeders (NetworkSeeder → zones.border) against the bundled MySQL 8 container. The same path breaks Zone creation through the API.

Changes

• Casts\Polygon::set() now returns new SpatialExpression($value) for SpatialPolygon and GeometryInterface inputs, mirroring MultiPolygon exactly (the GeoJSON and SpatialExpression branches are unchanged — non-object inputs never enter the cast cache, so the geometry-object return there is still correct).
• Updated SpatialCastBranchesTest which previously asserted the old hand-back-the-object behavior; it now expects a SpatialExpression like the other casts.

Test plan

• db:seed --class="Fleetbase\FleetOps\Seeders\Testing\TestingSeeder" completes on a fresh Docker install (was failing on zone insert)
• CI: unit tests