Fix __typename field handling to use GraphQLString

[smarongiu]

Fix __typename field handling to use GraphQLString instead of creating new scalar

Description

Fixes a bug where code generation fails with TypeError: Redefinition of reserved type 'String' when processing queries that include __typename fields. The issue occurs because __typename is a GraphQL meta-field that isn't explicitly defined in schema type maps, so the code falls back to creating a field definition. The original implementation incorrectly attempted to create a new GraphQLScalarType(name="String"), which fails because "String" is a reserved built-in type.

Minimal Example

Schema:

type Query {
  user: User
}

type User {
  id: ID!
  name: String!
}

Query:

query GetUser {
  user {
    __typename
    id
    name
  }
}

Before (fails):

return GraphQLField(
    type_=GraphQLNonNull(type_=GraphQLScalarType(name="String"))  # ❌ TypeError
)

After (works):

return GraphQLField(
    type_=GraphQLNonNull(type_=GraphQLString)  # ✅ Uses built-in type
)

Changes

• Added GraphQLString import to ariadne_codegen/client_generators/result_types.py
• Updated _get_field_from_schema() to use GraphQLString instead of creating a new scalar for __typename handling
• Added test case test_get_field_from_schema_handles_typename_correctly() to validate the fix

Testing

• All existing tests pass (6/6 in test_include_typename.py)
• New test validates __typename field handling and would fail without the fix

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• Code follows the project's style guidelines
• Tests pass locally
• Added tests for the fix
• No breaking changes introduced

[DamianCzajkowski]

Hi @smarongiu, nice pull request! Can you run the linters so the GitHub Actions aren't failing?