test(releases): port ReleaseClient + ChecksumVerifier tests to new project

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

tests/ClaudeDo.Releases.Tests/FakeHttpMessageHandler.cs

@@ -0,0 +1,32 @@
+using System.Net;
+using System.Net.Http;
+
+namespace ClaudeDo.Releases.Tests;
+
+internal sealed class FakeHttpMessageHandler : HttpMessageHandler
+{
+    private readonly Func<HttpRequestMessage, HttpResponseMessage> _handler;
+    private readonly object _lock = new();
+    private readonly List<HttpRequestMessage> _requests = new();
+
+    public FakeHttpMessageHandler(Func<HttpRequestMessage, HttpResponseMessage> handler)
+    {
+        _handler = handler;
+    }
+
+    public FakeHttpMessageHandler(HttpStatusCode status, string body)
+        : this(_ => new HttpResponseMessage(status) { Content = new StringContent(body) })
+    {
+    }
+
+    public IReadOnlyList<HttpRequestMessage> Requests
+    {
+        get { lock (_lock) return _requests.ToArray(); }
+    }
+
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        lock (_lock) _requests.Add(request);
+        return Task.FromResult(_handler(request));
+    }
+}