Server-side scenario execution: effects client replaces local engine

- client.Effects: typed effects + completion methods posting scenario callbacks
- client.Scenario reduced to TriggerAsync
- pending polling via Update() pump: 30s heartbeat + wait-deadline checks
- local engine, plan persistence (IPlanStateStore) and resume removed
- RealtimeService and realtime transports removed (relay never deployed)
- dead IPlanScheduler leftover removed
- models regenerated from openapi (ExecutionPlan/BoundaryNode gone, PendingEffect added)

Services/Effects/NotificationEffect.cs

@@ -0,0 +1,40 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Newtonsoft.Json.Linq;
+
+namespace RudderSdk.Core;
+
+/// <summary>A scenario notification node. <see cref="DoneAsync"/> posts the <c>output</c> callback.</summary>
+public sealed class NotificationEffect
+{
+    private readonly EffectHandle _handle;
+
+    internal NotificationEffect(EffectHandle handle) => _handle = handle;
+
+    /// <summary>Run id.</summary>
+    public string RunId => _handle.RunId;
+
+    /// <summary>Scenario id.</summary>
+    public string ScenarioId => _handle.ScenarioId;
+
+    /// <summary>Node id.</summary>
+    public string NodeId => _handle.NodeId;
+
+    /// <summary>Notification title.</summary>
+    public string Title => _handle.Get("title", string.Empty);
+
+    /// <summary>Notification message.</summary>
+    public string Message => _handle.Get("message", string.Empty);
+
+    /// <summary>Node data payload.</summary>
+    public JObject Data => _handle.Data;
+
+    /// <summary>Reads a typed value from the node data.</summary>
+    public T Get<T>(string key, T defaultValue = default!) => _handle.Get(key, defaultValue);
+
+    /// <summary>Posts the <c>output</c> callback.</summary>
+    public Task DoneAsync(CancellationToken cancellationToken = default) => _handle.CompleteAsync("output", cancellationToken);
+
+    /// <summary>Posts the <c>output</c> callback (fire-and-forget).</summary>
+    public void Done() => _handle.Complete("output");
+}