Dynamic parameterized Ecto Type for GraphQL union/interface types.

Dispatches to the correct embedded schema module based on the __typename field in the JSON map.

The generated module implements Ecto.ParameterizedType and uses embed_as: :dump so that Ecto.embedded_load/3 calls load/3, which reads __typename and delegates to the matched module via Ecto.embedded_load/3.

An unresolvable __typename returns :error, as the callback contract requires; Ecto turns that into its own ArgumentError naming the offending map, the field and the schema, so nothing is lost by not carrying a message.

cast/2 accepts the wire shape — a decoded JSON map with a string "__typename" — and reads the values inside the variant with load semantics, so an already-cast value such as an enum atom is not accepted.