Use short-term JWT instead of instance-level JWT for direct connections

In scope of #452044 (closed) the plan is to used instance-level JWT for direct connections, the only reason is to unblock frontend work (as for frontend it doesn't matter what JWT is used).

But before we enable direct connections globally, we need to make sure that a valid short-term JWTs are used.

The scope of this task is to replace the way how JWT is obtained on Rails server: instead of returning instance-level JWT, Rails should send a request to AI Gateway to create this JWT.

Nothing will change for IDE/client (except just JWT's expiration time).