test: cover starting a chat with a player end to end

Reproduces the two-step handshake the client performs — POST thread/withmembers
to create the thread, then GET thread/{id} to open it — on a conversation that
has never carried a message, which is the case that was 404ing.

The request is sent as a form body rather than a query string, matching the
wire ("ids=9741717&messageCount=50"). That detail is load-bearing for routing,
not just fidelity: POST thread/{chatThreadId} also matches the literal path
thread/withmembers, and the content type is what separates them. Posting with
no body makes the two indistinguishable and the request fails as an ambiguous
match across CreateGroup, WithMembersForm and SendMessageForm.

DorkNet.Server.Tests/ChatStartConversationTests.cs

@@ -0,0 +1,83 @@
+using System.Net;
+using System.Net.Http.Headers;
+using System.Text.Json;
+
+namespace DorkNet.Server.Tests;
+
+/// <summary>
+/// Starting a chat with a player is a two-step handshake, and both steps have to
+/// work on a thread that has never carried a message:
+///
+///   POST thread/withmembers?ids={playerId}   -> creates/returns the thread
+///   GET  thread/{id}                         -> opens it
+///
+/// Observed failing in production: the POST returned 200 at 06:38:20.862 and the
+/// GET 404'd 415ms later, because the handler decided a thread existed by
+/// counting its messages. A brand-new conversation has none, so the client was
+/// handed a thread id it was then told did not exist, and no chat could ever be
+/// opened with someone you had not already messaged.
+/// </summary>
+public sealed class ChatStartConversationTests : IClassFixture<DorkNetServerFactory>
+{
+    private readonly DorkNetServerFactory _factory;
+
+    public ChatStartConversationTests(DorkNetServerFactory factory) => _factory = factory;
+
+    [Fact]
+    public async Task Brand_new_conversation_can_be_opened_before_any_message_is_sent()
+    {
+        using var starterClient = Client();
+        using var otherClient = Client();
+
+        var starter = await GameClientSessionFactory.CreateAsync(starterClient, _factory.ApexDomain);
+        var other = await GameClientSessionFactory.CreateAsync(otherClient, _factory.ApexDomain);
+
+        starterClient.DefaultRequestHeaders.Authorization =
+            new AuthenticationHeaderValue("Bearer", starter.AccessToken);
+
+        // Step 1 — the client asks for the thread with that player. It sends the
+        // ids as a form body (observed on the wire as
+        // "ids=9741717&messageCount=50"), which also matters for routing: the
+        // content type is what separates this from POST thread/{chatThreadId},
+        // whose parameter segment otherwise also matches "withmembers".
+        using var form = new FormUrlEncodedContent(
+        [
+            new KeyValuePair<string, string>("ids", other.PlayerId.ToString()),
+            new KeyValuePair<string, string>("messageCount", "50"),
+        ]);
+        using var createResponse = await starterClient.PostAsync("/thread/withmembers", form);
+        var createBody = await createResponse.Content.ReadAsStringAsync();
+        Assert.True(createResponse.IsSuccessStatusCode,
+            $"POST /thread/withmembers -> {(int)createResponse.StatusCode}: {createBody}");
+
+        var thread = JsonDocument.Parse(createBody).RootElement;
+        Assert.True(thread.TryGetProperty("ChatThreadId", out var idElement)
+                    || thread.TryGetProperty("chatThreadId", out idElement),
+            $"no thread id in the withmembers reply: {createBody}");
+        var threadId = idElement.ValueKind == JsonValueKind.String
+            ? idElement.GetString()
+            : idElement.ToString();
+        Assert.False(string.IsNullOrWhiteSpace(threadId));
+
+        // Step 2 — and immediately opens it. No message has been sent yet.
+        using var openResponse = await starterClient.GetAsync($"/thread/{threadId}?maxCount=50");
+        var openBody = await openResponse.Content.ReadAsStringAsync();
+
+        Assert.True(openResponse.StatusCode != HttpStatusCode.NotFound,
+            $"""
+             GET /thread/{threadId} returned 404 for a thread that thread/withmembers
+             had just created. An empty thread is not a missing thread — the chat
+             cannot be opened until someone has already sent a message, which is
+             impossible to do first.
+             """);
+        Assert.True(openResponse.IsSuccessStatusCode,
+            $"GET /thread/{threadId} -> {(int)openResponse.StatusCode}: {openBody}");
+    }
+
+    private HttpClient Client()
+    {
+        var client = _factory.CreateClient(new() { AllowAutoRedirect = false });
+        client.BaseAddress = new Uri($"http://chat.{_factory.ApexDomain}");
+        return client;
+    }
+}