Fix graphql tool (#4984)

Fix construction and add unit test.

tests/integration_tests/utilities/test_graphql.py

@@ -1,32 +0,0 @@
-import json
-
-import pytest
-import responses
-
-from langchain.utilities.graphql import GraphQLAPIWrapper
-
-TEST_ENDPOINT = "http://testserver/graphql"
-
-# Mock GraphQL response for testing
-MOCK_RESPONSE = {
-    "data": {"allUsers": [{"id": 1, "name": "Alice", "email": "alice@example.com"}]}
-}
-
-
-@pytest.fixture
-def graphql_wrapper() -> GraphQLAPIWrapper:
-    return GraphQLAPIWrapper(
-        graphql_endpoint=TEST_ENDPOINT,
-        custom_headers={"Authorization": "Bearer testtoken"},
-    )
-
-
-@responses.activate
-def test_run(graphql_wrapper: GraphQLAPIWrapper) -> None:
-    responses.add(responses.POST, TEST_ENDPOINT, json=MOCK_RESPONSE, status=200)
-
-    query = "query { allUsers { id, name, email } }"
-    result = graphql_wrapper.run(query)
-
-    expected_result = json.dumps(MOCK_RESPONSE, indent=2)
-    assert result == expected_result