feat(mission): /mission/receive_reward stub (refresh-only, no decomp)

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

SVSim.UnitTests/Controllers/MissionControllerReceiveRewardTests.cs

@@ -0,0 +1,26 @@
+using System.Net;
+using System.Text;
+using NUnit.Framework;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class MissionControllerReceiveRewardTests
+{
+    [Test]
+    public async Task ReceiveReward_returns_ok()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(tutorialState: 0);
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        // INFERRED: id of the mission to claim. Defensive stub accepts even when
+        // the id doesn't match a current mission — there is no decomp Parse() to
+        // tell us how the server errors.
+        var requestJson = """{"id":1,"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
+        var response = await client.PostAsync("/mission/receive_reward",
+            new StringContent(requestJson, Encoding.UTF8, "application/json"));
+
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+    }
+}