feat(tutorial): add /tutorial/update_action fire-and-forget endpoint

Returns an empty data object (result_code=1 from middleware envelope).
Client uses SkipAllNetworkChecks so the response body is never read.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.UnitTests/Controllers/TutorialControllerTests.cs

@@ -0,0 +1,36 @@
+using System.Net;
+using System.Text;
+using System.Text.Json;
+using NUnit.Framework;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class TutorialControllerTests
+{
+    [Test]
+    public async Task UpdateAction_returns_result_code_1_with_empty_data()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(tutorialState: 0);
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        // tutorial_step and tutorial_action_number are fire-and-forget bookkeeping fields;
+        // send representative values from the live capture (step=1, action=2).
+        var requestJson =
+            """{"tutorial_step":1,"tutorial_action_number":2,"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
+
+        var response = await client.PostAsync("/tutorial/update_action",
+            new StringContent(requestJson, Encoding.UTF8, "application/json"));
+
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+        var body = await response.Content.ReadAsStringAsync();
+
+        // Controllers return the INNER data payload; envelope is middleware's job.
+        // For the no-op shape the action returns an empty object.
+        using var doc = JsonDocument.Parse(body);
+        Assert.That(doc.RootElement.ValueKind, Is.EqualTo(JsonValueKind.Object));
+        Assert.That(doc.RootElement.EnumerateObject().Count(), Is.EqualTo(0),
+            "update_action returns empty data — client uses SkipAllNetworkChecks and reads nothing.");
+    }
+}