Add integration tests with real GraphQL servers

[pgatzka]

User Story

As a maintainer, I want integration tests against real GraphQL servers so that I can verify compatibility with production environments.

Description

Create integration test suite that tests against real GraphQL servers:

Servers to test:

• Apollo Server (Node.js)
• graphql-java (Spring GraphQL)
• Hasura (if feasible with Docker)

Test scenarios:

• Queries with various field types
• Mutations with input validation
• Error handling scenarios
• Authentication flows
• Subscriptions (if supported)

Acceptance Criteria

• Integration tests run against Apollo Server
• Integration tests run against graphql-java
• Tests cover queries, mutations, subscriptions
• Tests cover error scenarios
• Docker Compose setup for test servers
• CI integration with test containers
• Implemented code is covered with automated tests
• Documentation has been updated