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/WaitEffect.cs

@@ -0,0 +1,34 @@
+using System;
+using Newtonsoft.Json.Linq;
+
+namespace RudderSdk.Core;
+
+/// <summary>A scenario wait node. The run resumes when the server advances it at <see cref="DeadlineUtc"/>.</summary>
+public sealed class WaitEffect
+{
+    private readonly EffectHandle _handle;
+
+    internal WaitEffect(EffectHandle handle, DateTimeOffset deadlineUtc)
+    {
+        _handle = handle;
+        DeadlineUtc = deadlineUtc;
+    }
+
+    /// <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>When the wait ends (UTC).</summary>
+    public DateTimeOffset DeadlineUtc { get; }
+
+    /// <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);
+}