Add check for existing env vars in backend templates

[cedricc13]

What does this PR do?

• Adds a helper function envHasName to check if an environment variable is already defined in the backend env list.
• Uses this function in backend-statefulset.yaml to avoid adding duplicate environment variables.
• Fixes Helm template errors when backend.env is a list.

Why is this needed?

• Helm templates failed because .Values.langflow.backend.env is a list, not a map.
• Prevents overwriting or duplicating environment variables.

How to test?

• Run helm template with updated values.yaml to verify no duplicate env vars.

[flefevre]

Seems good and will be very useful.

[blegry]

Yes, need this update !