Support API key configuration for Zerox in multi-tenant setup

We are currently using Zerox for document transformation in our system. However, Zerox only supports reading the API key from environment variables.

Our platform is multi-tenant, and the API key is scoped per project. This makes it problematic to rely solely on environment variables for API key management.

We already opened a PR in the Zerox repo to add support for passing the API key explicitly instead of always fetching it from environment variables. However, in case we don’t receive a response from the maintainers, we will need to either:

• Move to another tool that supports this flexibility, or
• Implement a workaround where each Celery worker handles project-specific API keys.

Context:

Each Celery worker runs in a separate process, which means separate memory and environment, so we can set api_key in env.

Test:
Once fixed this add unit tests for transfomer

Comienza localizando la integración de Zerox y el área de pruebas unitarias del transformer en el backend de Python. Comprueba si el PR de API-key upstream está disponible y, después, define cómo deben llegar las claves específicas del proyecto al transformer; se considera terminado cuando el enfoque seleccionado funciona para el uso multi-tenant y existen pruebas unitarias para el transformer.