PostIt/Yavsc.Blogs: surface 4xx body + pin controller + red UI test for Save

The 'Save' button in PostIt has been returning 400 from
/api/v1/blog ever since the editor's title and article fields
were re-bound to SelectedPost.Title / SelectedPost.Article.
The user types into the editor, taps Save, the controller
rejects with 'The Title field is required', and the PostIt
status bar shows only the generic 'Response status code does
not indicate success: 400' — no field name, no reason.

Three pieces here make the regression diagnosable and pin a
test for the fix:

1. YavscApiClient: replace EnsureSuccessStatusCode() at both
   call sites with a small helper that reads the response
   body and embeds it in the thrown HttpRequestException. The
   VM's existing catch (Exception) in ExecuteAsync forwards
   ex.Message to the status bar, so the next 'click Save'
   tells the user exactly which field the server rejected.

2. Yavsc.Blogs.Tests: two integration tests on the real
   controller (no HTTP mock) — one pins that a well-formed
   PostIt-shaped payload (Title + Article + AuthorId + dates,
   Id=0) is accepted with 201, the other pins that a payload
   with Title=string.Empty is rejected with 400. Together they
   pin the contract the VM has to honour.

3. PostIt.Tests: a red [AvaloniaFact] UI test that mounts
   MainPage inside a headless Window, types a title into the
   TextBox without first selecting a post in the list, taps
   Save, and asserts the body of the first POST contains
   the typed title. Today this test fails with Title='',
   reproducing the production 400. The matching fix (a
   Title/Article buffer on MainPageViewModel that the XAML
   binds to, and that Save uses to build the outgoing
   BlogPost) is the next commit; the test is the safety net.

src/PostIt.Tests/BlogApiTestFakes.cs

@@ -0,0 +1,65 @@
+using PostIt.Models;
+using PostIt.Services;
+using PostIt.ViewModels;
+
+namespace PostIt.Tests;
+
+/// <summary>Per-call ledger shared between the test and the
+/// recording fake, so the assertion can inspect what the VM
+/// actually sent on the wire without coupling to the fake's
+/// internals.</summary>
+internal sealed class CallRecorder
+{
+    public (HttpMethod method, string path, object? body) FirstCall =>
+        Calls[0];
+    public List<(HttpMethod method, string path, object? body)> Calls { get; } = new();
+}
+
+/// <summary>Test fake that records every CallAsync invocation
+/// and answers them with a canned sequence: the first call gets
+/// a server-issued BlogPost (Id=42), the second call gets a
+/// single-element list containing that post. Used by the ViewModel
+/// tests and the headless UI test to capture exactly what the
+/// Save button posts to the server.</summary>
+internal sealed class RecordingYavscApiClient : YavscApiClient
+{
+    private readonly CallRecorder _recorder;
+    public RecordingYavscApiClient(CallRecorder recorder)
+        : base(
+            new Settings
+            {
+                Authentication = new AuthenticationSettings
+                {
+                    Authority = "https://stub.invalid",
+                    ClientId = "stub",
+                    Scopes = new[] { "openid" },
+                },
+            },
+            new TokenStore(System.IO.Path.GetTempFileName()))
+    {
+        _recorder = recorder;
+    }
+
+    public override Task<T> CallAsync<T>(HttpMethod method, string path, object? body = null, CancellationToken ct = default)
+    {
+        _recorder.Calls.Add((method, path, body));
+        // BlogPost? boxes to BlogPost at runtime, so we test the
+        // non-nullable type — typeof(BlogPost?) is a C# error
+        // (CS8639: "typeof cannot be used on a nullable reference
+        // type").
+        if (typeof(T) == typeof(BlogPost))
+            return Task.FromResult((T)(object)new BlogPost
+            {
+                Id = 42,
+                Title = "Mon premier billet",
+                AuthorId = "tester",
+                Article = "Contenu du billet de test.",
+            });
+        if (typeof(T) == typeof(List<BlogPost>))
+            return Task.FromResult((T)(object)new List<BlogPost>
+            {
+                new() { Id = 42, Title = "Mon premier billet" }
+            });
+        return Task.FromResult(default(T)!);
+    }
+}