Add feature flag to send a nil prompt to the AI gateway.
What does this MR do and why?
In this MR we:
Add ai_custom_models_prompts_migration FF
- Add a feature flag to control prompts migration
- Use the new FF to route to
AiGatewayCodeCompletionMessage
- Adjust the base prompt in
AiGatewayCodeCompletionMessage
to be nil
How to set up and validate locally
- With the feature flag disabled, verify that the prompt is fully sent to the AI gateway and that the completion request succeeds.
- With the feature flag enabled, verify that the prompt is sent as
None
to the AI gateway and that the completion request succeeds.
Related to #473156 (closed)