fix: an empty chat thread is not a missing chat thread

Starting a chat with a player POSTs thread/withmembers (which creates the
thread) and then immediately GETs thread/{id}. At that instant the thread has
no messages, and GetThreadById returned NotFound purely on message count — so
the client got a 404 for the thread it had just been handed and the
conversation could never be opened.

Confirmed from a server log: POST /thread/withmembers -> 200 at 06:38:20.862,
GET /thread/15 -> 404 at 06:38:21.277.

The handler now 404s only when nothing records the thread — no ChatThreads row
and no ChatThreadMembers rows — and otherwise returns it with an empty message
list.

Also adds RoomCloneShapeTests. The 2023 copy-room flow reads the clone reply
into FGCPNAACHIK, the same type get-by-id returns
(NLDBPDCNNCF.txt:4676 signature, :4802 route literal), so the two responses
must stay shape-identical: if they drift, the server still answers 200 and the
client throws while reading, surfacing much later from the finalizer as a bare
"Failed to copy room" with nothing in the server log. The test compares both
top-level and nested sub-room keys, and seeds a real scene row so the
comparison exercises the populated branch rather than the synthesised-default
fallback that a room with no scenes would take.

DorkNet.Server.Tests/RoomCloneShapeTests.cs

@@ -0,0 +1,119 @@
+using System.Net.Http.Headers;
+using System.Text.Json;
+using DorkNet.Server.Data;
+using DorkNet.Server.Data.Entities;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DorkNet.Server.Tests;
+
+/// <summary>
+/// The 2023 "copy room" flow POSTs <c>rooms/{id}/clone</c> and deserialises the
+/// reply into <c>FGCPNAACHIK</c> — the SAME type <c>GET rooms/{id}</c> returns
+/// (RecNet.Runtime/NLDBPDCNNCF.txt:4676 declares
+/// <c>Task&lt;FGCPNAACHIK&gt; GDHIIAHCBMN(Int64, String, CancellationToken)</c>,
+/// with the route literal on :4802).
+///
+/// So the two responses must be shape-identical. When they drift, the server
+/// still answers 200 and the client throws while reading it — and because the
+/// clone promise's exception is never observed, it surfaces from the finalizer
+/// thread seconds later as a bare "Failed to copy room: Exception of type
+/// '...' was thrown", with nothing in the server log to show for it. That is
+/// exactly how this failed in the wild: the room WAS created, and the client
+/// simply never registered it.
+/// </summary>
+public sealed class RoomCloneShapeTests : IClassFixture<DorkNetServerFactory>
+{
+    private readonly DorkNetServerFactory _factory;
+
+    public RoomCloneShapeTests(DorkNetServerFactory factory) => _factory = factory;
+
+    [Fact]
+    public async Task Clone_response_matches_the_get_by_id_shape()
+    {
+        using var client = Client();
+        var session = await GameClientSessionFactory.CreateAsync(client, _factory.ApexDomain);
+        client.DefaultRequestHeaders.Authorization =
+            new AuthenticationHeaderValue("Bearer", session.AccessToken);
+
+        var sourceId = 9_400_000 + Random.Shared.Next(1, 99_999);
+        await using (var scope = _factory.Services.CreateAsyncScope())
+        {
+            var db = scope.ServiceProvider.GetRequiredService<DorkNetDbContext>();
+            db.Rooms.Add(new RoomEntity
+            {
+                Id = sourceId,
+                Name = $"CloneSrc{Guid.NewGuid():N}"[..20],
+                CreatorPlayerId = session.PlayerId,
+                CloningAllowed = true,
+            });
+            // Give the source a real sub-room. Without one, BOTH responses fall
+            // back to the synthesised default sub-room and the comparison never
+            // exercises the branch a real room (like the seeded RecCenter the
+            // room-creation picker clones) actually takes.
+            db.RoomScenes.Add(new RoomSceneEntity
+            {
+                RoomId = sourceId,
+                Name = "Home",
+                OrderIndex = 0,
+            });
+            await db.SaveChangesAsync();
+        }
+
+        var getBody = await ReadJsonAsync(client, HttpMethod.Get, $"/rooms/{sourceId}");
+        var cloneBody = await ReadJsonAsync(client, HttpMethod.Post, $"/rooms/{sourceId}/clone",
+            new FormUrlEncodedContent([new KeyValuePair<string, string>("name", $"Copy{Guid.NewGuid():N}"[..20])]));
+
+        var expected = getBody.EnumerateObject().Select(p => p.Name).ToHashSet(StringComparer.Ordinal);
+        var actual = cloneBody.EnumerateObject().Select(p => p.Name).ToHashSet(StringComparer.Ordinal);
+
+        var missing = expected.Except(actual).Order(StringComparer.Ordinal).ToList();
+
+        // The reader walks the nested sub-room list too, so a key missing there
+        // breaks it just as hard as one missing at the top level.
+        foreach (var list in new[] { "SubRooms", "Scenes" })
+        {
+            if (!getBody.TryGetProperty(list, out var expectedList) ||
+                expectedList.ValueKind != JsonValueKind.Array ||
+                expectedList.GetArrayLength() == 0) continue;
+
+            Assert.True(cloneBody.TryGetProperty(list, out var actualList)
+                        && actualList.ValueKind == JsonValueKind.Array,
+                $"clone response has no {list} array while get-by-id does");
+            Assert.True(actualList.GetArrayLength() > 0,
+                $"clone response has an EMPTY {list} array while get-by-id returns "
+                + $"{expectedList.GetArrayLength()}; the client's reader requires at least one entry");
+
+            var expectedKeys = expectedList[0].EnumerateObject().Select(p => p.Name);
+            var actualKeys = actualList[0].EnumerateObject().Select(p => p.Name).ToHashSet(StringComparer.Ordinal);
+            missing.AddRange(expectedKeys.Where(k => !actualKeys.Contains(k)).Select(k => $"{list}[0].{k}"));
+        }
+        Assert.True(missing.Count == 0,
+            $"""
+             The clone response is missing {missing.Count} key(s) that get-by-id
+             returns. Both deserialize into FGCPNAACHIK, so the client throws while
+             reading the clone and silently never registers the new room:
+
+             {string.Join(Environment.NewLine, missing)}
+
+             clone keys: {string.Join(", ", actual.Order(StringComparer.Ordinal))}
+             """);
+    }
+
+    private static async Task<JsonElement> ReadJsonAsync(
+        HttpClient client, HttpMethod method, string path, HttpContent? content = null)
+    {
+        using var request = new HttpRequestMessage(method, path) { Content = content };
+        using var response = await client.SendAsync(request);
+        var body = await response.Content.ReadAsStringAsync();
+        Assert.True(response.IsSuccessStatusCode,
+            $"{method} {path} -> {(int)response.StatusCode}: {body}");
+        return JsonDocument.Parse(body).RootElement.Clone();
+    }
+
+    private HttpClient Client()
+    {
+        var client = _factory.CreateClient(new() { AllowAutoRedirect = false });
+        client.BaseAddress = new Uri($"http://rooms.{_factory.ApexDomain}");
+        return client;
+    }
+}