[cli] prevent ot-ctl termination during debug command (#12600)

Under interactive mode, the `ot-ctl` client treats lines starting with
"Error" as fatal command failures. It exits immediately and stop
receiving CLI output. As the `debug` command runs a sequence of
sub-commands; if one fails , the entire debug session would stop.

This change modifies the error prefix to "ERROR" for internal debug
commands, allowing ot-ctl to continue processing subsequent output.
Also, it adds an explicit `OutputLine("Done")` at the end of the debug
command processing to ensure the CLI client correctly detects the end
of the command.

src/cli/cli.cpp

@@ -164,7 +164,9 @@ void Interpreter::OutputResult(otError aError)
     {
         if (aError != OT_ERROR_NONE)
         {
-            OutputLine("Error %u: %s", aError, otThreadErrorToString(aError));
+            // For internal debug commands, prepending `*` to prevent cli client from treating this as a fatal error and
+            // exit.
+            OutputLine("* Error %u: %s", aError, otThreadErrorToString(aError));
         }
 
         ExitNow();