Make sure polygons contain valid rings

According to the GeoJSON specification [1]:

> * A linear ring is a closed LineString with four or more positions.
> * The first and last positions are equivalent, and they MUST contain
>   identical values; their representation SHOULD also be identical.
> (...)
> * For type "Polygon", the "coordinates" member MUST be an array of
>   linear ring coordinate arrays.

Note that, for simplicity, right now we aren't checking whether the
coordinates are defined counterclockwise for exterior rings and
clockwise for interior rings, which is what the specification expects.

[1] https://datatracker.ietf.org/doc/html/rfc7946#section-3.1.6

app/models/concerns/geojson_format_validator.rb

@@ -115,8 +115,13 @@ class GeojsonFormatValidator < ActiveModel::EachValidator
     end
 
     def valid_polygon_coordinates?(polygon_coordinates)
-      polygon_coordinates.all? do |ring_coordinates|
-        valid_coordinates_array?(ring_coordinates)
-      end
+      polygon_coordinates.is_a?(Array) &&
+        polygon_coordinates.all? { |ring_coordinates| valid_ring_coordinates?(ring_coordinates) }
+    end
+
+    def valid_ring_coordinates?(ring_coordinates)
+      valid_coordinates_array?(ring_coordinates) &&
+        ring_coordinates.size >= 4 &&
+        ring_coordinates.first == ring_coordinates.last
     end
 end