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/StoreOfferEffect.cs

@@ -0,0 +1,74 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Newtonsoft.Json.Linq;
+
+namespace RudderSdk.Core;
+
+/// <summary>A scenario store-offer node. Resolve it with <see cref="PurchaseAsync"/> or <see cref="DeclineAsync"/>.</summary>
+public sealed class StoreOfferEffect
+{
+    private readonly EffectHandle _handle;
+
+    internal StoreOfferEffect(EffectHandle handle) => _handle = handle;
+
+    /// <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>Store slug from the node data, if present.</summary>
+    public string StoreSlug => _handle.Get("storeSlug", string.Empty);
+
+    /// <summary>Optional message from the node data.</summary>
+    public string? Message
+    {
+        get
+        {
+            var value = _handle.Get<string?>("message", null);
+            return string.IsNullOrEmpty(value) ? null : value;
+        }
+    }
+
+    /// <summary>Node data payload.</summary>
+    public JObject Data => _handle.Data;
+
+    /// <summary>True after the offer was resolved once.</summary>
+    public bool IsResolved { get; private set; }
+
+    /// <summary>Reads a typed value from the node data.</summary>
+    public T Get<T>(string key, T defaultValue = default!) => _handle.Get(key, defaultValue);
+
+    /// <summary>Posts the <c>onPurchase</c> callback.</summary>
+    public Task PurchaseAsync(CancellationToken cancellationToken = default) => ResolveAsync("onPurchase", cancellationToken);
+
+    /// <summary>Posts the <c>onPurchase</c> callback (fire-and-forget).</summary>
+    public void Purchase() => Resolve("onPurchase");
+
+    /// <summary>Posts the <c>onDecline</c> callback.</summary>
+    public Task DeclineAsync(CancellationToken cancellationToken = default) => ResolveAsync("onDecline", cancellationToken);
+
+    /// <summary>Posts the <c>onDecline</c> callback (fire-and-forget).</summary>
+    public void Decline() => Resolve("onDecline");
+
+    private async Task ResolveAsync(string handle, CancellationToken cancellationToken)
+    {
+        if (IsResolved)
+            return;
+
+        IsResolved = true;
+        await _handle.CompleteAsync(handle, cancellationToken).ConfigureAwait(false);
+    }
+
+    private void Resolve(string handle)
+    {
+        if (IsResolved)
+            return;
+
+        IsResolved = true;
+        _handle.Complete(handle);
+    }
+}