feat: Notify sbtn client when command is queued

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

Changes

• NetworkChannel.scala: Added notifyIfQueued() method that checks StandardMain.exchange.currentExec and sends a notification if a command is running
• NetworkClient.scala: Added handler for "sbt/exec" notifications with "Queued" status to print the waiting message to stderr

Testing

• Added QueuedNotificationTest with test project in server-test/src/server-test/queued/

Checklist

• Compiles successfully
• scalafmtCheck passes
• mimaReportBinaryIssues passes (binary compatible)
• Tests added

Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=89318445