Introduces a comprehensive integration testing setup and local development environment.
Changes include:
- Add `docker-compose.yml` for local stack orchestration.
- Add `docker-compose.integration.yml` for the integration test environment.
- Create `mock-upstream` service to simulate external LLM provider responses.
- Implement Go-based end-to-end tests verifying control plane configuration and data plane routing.
- Add `integration_test.sh` for quick connectivity verification.
