When a new builder is added for flutter repositories, we need to update below two config files sequentially:

1. add a builder configuration entry in flutter/infra
2. enable the builder in try_builder.json or prod_builder.json.

If a builder is enabled before the builder gets configured in flutter/infra, it will not be triggered and scheduled to run.

This has been causing cocoon backend failures and not-up-to-date tree status: #79380

We need to create a test to validate existence of the configuration before enabling in the json files.