feat(http): ArenaTwoPickBattleController (do_matching stub + finish)

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

SVSim.UnitTests/Controllers/ArenaTwoPickBattleControllerTests.cs

@@ -0,0 +1,24 @@
+using System.Net;
+using System.Net.Http.Json;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class ArenaTwoPickBattleControllerTests
+{
+    [Test]
+    public async Task DoMatching_returns_perpetual_searching_with_empty_node_url()
+    {
+        using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+        var req = new { deck_no = 1L, need_init = 1, log = 1, excluded_field_id_list = new long[] { }, use_stage_select = 1, is_default_skin = 0 };
+        var resp = await client.PostAsync("/arena_two_pick_battle/do_matching", JsonContent.Create(req));
+
+        Assert.That(resp.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+        var body = await resp.Content.ReadAsStringAsync();
+        StringAssert.Contains("\"matching_state\":3002", body);
+        StringAssert.Contains("\"node_server_url\":\"\"", body);
+        StringAssert.DoesNotContain("\"battle_id\"", body);
+    }
+}