Issue #10: implement worker process launcher

src/MxGateway.Server/Workers/WorkerProcessStartedProbe.cs

@@ -0,0 +1,19 @@
+namespace MxGateway.Server.Workers;
+
+public sealed class WorkerProcessStartedProbe : IWorkerStartupProbe
+{
+    public Task WaitUntilReadyAsync(
+        IWorkerProcess process,
+        WorkerProcessLaunchRequest request,
+        CancellationToken cancellationToken)
+    {
+        if (process.HasExited)
+        {
+            throw new WorkerProcessLaunchException(
+                WorkerProcessLaunchErrorCode.StartupFailed,
+                $"Worker process exited before startup completed with exit code {process.ExitCode}.");
+        }
+
+        return Task.CompletedTask;
+    }
+}