Fix sync_response output for ONCE mode subscriptions

[earies]

https://github.com/openconfig/reference/blob/master/rpc/gnmi/gnmi-specification.md#35151-once-subscriptions

Current behavior was suppressing sync_response output for ONCE mode
subscriptions due to lack of switch/case fallthrough.

Collapsed switch/case to provide same behavior on logging as STREAM

[earies]

Example output:

$ ./gnmic -a r1:9339 -u user -p pass! --insecure subscribe -e proto --format prototext --mode once --stream-mode sample --path /system/state/hostname
update: {
  timestamp: 1753007645075333719
  update: {
    path: {
      elem: {
        name: "system"
      }
      elem: {
        name: "state"
      }
      elem: {
        name: "hostname"
      }
    }
    val: {
      string_val: "r1"
    }
  }
}

sync_response: true

[earies]

@karimra bump for review/comment

[karimra]

Thanks for the contribution and sorry for the delay