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)

Scenarios/ListPendingScenarioEffectsResponse.cs

@@ -0,0 +1,12 @@
+// Code generated by apigen. DO NOT EDIT.
+using Newtonsoft.Json;
+using System.Collections.Generic;
+
+namespace RudderSdk.Core.Models.Scenarios;
+
+public class ListPendingScenarioEffectsResponse
+{
+    [JsonProperty("effects")]
+    public List<PendingEffect> Effects { get; set; } = null!;
+
+}