Write a SpacetimeDB backend module in C# that defines a sum type and uses it in multiple table columns.

TYPES
- Struct: Circle
  - Fields:
    - Radius: int
- Struct: Rectangle
  - Fields:
    - Width: int
    - Height: int
- Sum: Shape = Circle | Rectangle

TABLE
- Drawing
  - Struct: Drawing
  - Fields:
    - Id: int (primary key)
    - A: Shape
    - B: Shape

REDUCERS
- Seed: insert exactly one row into Drawing
  - (Id=1, A=Circle{Radius=10}, B=Rectangle{Width=4, Height=6})
