[2.x] feat: Notify sbtn client when command is queued (#8568)

Fixes #8356

**Problem**

When `sbtn` sends a command while another long-running task (like `console`) is already executing, the client silently blocks with no indication that the command is waiting in a queue.

**Solution**

When a new command arrives via the network channel and another command is currently running, the server now sends an `ExecStatusEvent` notification with status `"Queued"` to the client. The client displays a message like:

```
[info] waiting for: console
```

main-command/src/main/scala/sbt/internal/client/NetworkClient.scala

@@ -691,6 +691,14 @@ class NetworkClient(
           }
         case (`Shutdown`, Some(_))                => Vector.empty
         case (msg, _) if msg.startsWith("build/") => Vector.empty
+        case ("sbt/exec", Some(json)) =>
+          import sbt.protocol.codec.JsonProtocol.given
+          Converter.fromJson[ExecStatusEvent](json) match {
+            case Success(event) if event.status == "Queued" =>
+              event.message.foreach(m => errorStream.println(s"[info] $m"))
+              Vector.empty
+            case _ => Vector.empty
+          }
         case _ =>
           Vector(
             (