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)

Abstractions/IClock.cs

@@ -2,7 +2,7 @@ using System;
 
 namespace RudderSdk.Core.Abstractions;
 
-/// <summary>Time source used by the scenario runtime; override in tests.</summary>
+/// <summary>Time source used by the effects client; override in tests.</summary>
 public interface IClock
 {
     /// <summary>Current UTC time.</summary>