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/ScenarioFailedEffect.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace RudderSdk.Core;
+
+/// <summary>Payload of <see cref="EffectsService.OnScenarioFailed"/>.</summary>
+public sealed class ScenarioFailedEffect
+{
+    internal ScenarioFailedEffect(string runId, string scenarioId, string nodeId, Exception exception)
+    {
+        RunId = runId;
+        ScenarioId = scenarioId;
+        NodeId = nodeId;
+        Exception = exception;
+    }
+
+    /// <summary>Server-issued run id.</summary>
+    public string RunId { get; }
+
+    /// <summary>Scenario id.</summary>
+    public string ScenarioId { get; }
+
+    /// <summary>Node the failure happened at.</summary>
+    public string NodeId { get; }
+
+    /// <summary>The error that failed the run.</summary>
+    public Exception Exception { get; }
+}