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/UpdateScenarioCounterResponse.cs

@@ -1,6 +1,6 @@
 // Code generated by apigen. DO NOT EDIT.
 using Newtonsoft.Json;
-using RudderSdk.Core.Models;
+using Newtonsoft.Json.Linq;
 
 namespace RudderSdk.Core.Models.Scenarios;
 
@@ -9,7 +9,7 @@ public class UpdateScenarioCounterResponse
     [JsonProperty("completed")]
     public bool? Completed { get; set; }
 
-    [JsonProperty("plan")]
-    public ExecutionPlan? Plan { get; set; }
+    [JsonProperty("effect")]
+    public JToken? Effect { get; set; }
 
 }