Add failing test: duplicate typename__ when type implements multiple interfaces

[magicmark]

When a concrete type implements multiple interfaces and a query spreads on both with __typename, codegen emits the field twice — which makes mypy blow up with Name "typename__" already defined [no-redef].

type Foo implements Bar & Baz { id: ID!, name: String! }

query GetFoo {
    foo {
        ... on Bar { __typename id }
        ... on Baz { __typename name }
    }
}

Actual:

class GetFooFoo(BaseModel):
    typename__: Literal["Foo"] = Field(alias="__typename")
    id: str
    typename__: Literal["Foo"] = Field(alias="__typename")
    name: str

Expected:

class GetFooFoo(BaseModel):
    typename__: Literal["Foo"] = Field(alias="__typename")
    id: str
    name: str

Works fine at runtime (pydantic is happy), but mypy won't accept it.

Fix is at magicmark#1 (on my fork for now — can't target a fork branch as base on upstream).

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[magicmark]

@mirumee any thoughts?