#3703 fix(controlplane): reject streaming RPCs the server does not implement

Refs PFM-7992

Summary

The control plane's gRPC server implements only unary RPCs. gRPC reflection (a local-development convenience that is disabled in production) is the only streaming service it serves, introduced in #3694.

This adds a stream interceptor that returns Unimplemented for any other streaming method instead of dispatching it to a handler, so the server does not expose streaming endpoints it does not implement. Clients use unary RPCs and the HTTP endpoints, so there is no functional change for them.