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

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

SVSim.UnitTests/Controllers/MissionControllerBuyAdditionalRightTests.cs

@@ -0,0 +1,25 @@
+using System.Net;
+using System.Text;
+using NUnit.Framework;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class MissionControllerBuyAdditionalRightTests
+{
+    [Test]
+    public async Task BuyAdditionalRight_returns_ok()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(tutorialState: 0);
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        // Body shape is INFERRED — we send the bare base envelope and expect the server
+        // to refresh MissionInfoDetail.
+        var requestJson = """{"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
+        var response = await client.PostAsync("/mission/buy_additional_right",
+            new StringContent(requestJson, Encoding.UTF8, "application/json"));
+
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+    }
+}