Fix missing schema's ast_node when schema was defined inside extension

Replicates graphql/graphql-js@e2dec84

Author: Cito

graphql/utilities/extend_schema.py

@@ -373,7 +373,7 @@ def resolve_type(type_name: str) -> GraphQLNamedType:
         ),
         types=list(type_map.values()),
         directives=get_merged_directives(),
-        ast_node=schema.ast_node,
+        ast_node=schema_def or schema.ast_node,
         extension_ast_nodes=(
             schema.extension_ast_nodes or cast(Tuple[SchemaExtensionNode], ())
         )

tests/utilities/test_extend_schema.py

@@ -1082,16 +1082,17 @@ def adds_schema_definition_missing_in_the_original_schema():
             schema = GraphQLSchema(directives=[FooDirective], types=[FooType])
             assert schema.query_type is None
 
-            ast = parse(
+            extension_sdl = dedent(
                 """
                 schema @foo {
                   query: Foo
                 }
                 """
             )
-            schema = extend_schema(schema, ast)
+            schema = extend_schema(schema, parse(extension_sdl))
             query_type = schema.query_type
             assert query_type.name == "Foo"
+            assert print_ast(schema.ast_node) == extension_sdl.rstrip()
 
         def adds_new_root_types_via_schema_extension():
             schema = extend_test_schema(