mooclet UI: suppress Reward Metrics for other experiments from showing

In the Experiment stepper, in a mooclet experiment, when selecting a metric key for setting up queries, all of the reward metric keys available are showing.

Reward Metric keys should always only belong to one experiment.

<img width="1026" height="718" alt="Image" src="https://github.com/user-attachments/assets/a7fe8496-38be-4888-b849-330ad0809d67" />