Scaled down and converted images before sending them to AI models (#1101)

app/Tests/Tools/FakeImageRuntime.cs

@@ -0,0 +1,60 @@
+using System.Collections.Concurrent;
+using System.Net;
+using System.Text;
+using System.Text.Json;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Plays the image route of the runtime for a test, and notes what the app asked for.
+/// </summary>
+/// <remarks>
+/// It prepares nothing: every image comes back as the same three bytes, with the code and type the
+/// test sets. The answer is spelled out in the wire format of the runtime, so a test fails as soon
+/// as the app stops understanding what the runtime actually sends.
+/// </remarks>
+internal sealed class FakeImageRuntime : HttpMessageHandler
+{
+    /// <summary>
+    /// The three bytes every prepared image consists of, as base64.
+    /// </summary>
+    public const string IMAGE_BASE64 = "AQID";
+
+    private readonly ConcurrentQueue<string> requestedPaths = new();
+    private readonly ConcurrentQueue<long> requestedBudgets = new();
+
+    /// <summary>
+    /// The code the runtime answers with, as the runtime spells it.
+    /// </summary>
+    public string ErrorCode { get; set; } = "NONE";
+
+    /// <summary>
+    /// The MIME type the runtime reports for every prepared image.
+    /// </summary>
+    public string MimeType { get; set; } = "image/jpeg";
+
+    /// <summary>
+    /// The paths of the images the app asked to prepare, in order.
+    /// </summary>
+    public IReadOnlyList<string> RequestedPaths => [..this.requestedPaths];
+
+    /// <summary>
+    /// The byte budgets the app asked for, in order.
+    /// </summary>
+    public IReadOnlyList<long> RequestedBudgets => [..this.requestedBudgets];
+
+    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        if (request.RequestUri!.AbsolutePath is not "/image/prepare-for-model")
+            return new HttpResponseMessage(HttpStatusCode.NotFound);
+
+        using (var body = JsonDocument.Parse(await request.Content!.ReadAsStringAsync(cancellationToken)))
+        {
+            this.requestedPaths.Enqueue(body.RootElement.GetProperty("path").GetString() ?? string.Empty);
+            this.requestedBudgets.Enqueue(body.RootElement.GetProperty("max_bytes").GetInt64());
+        }
+
+        var answer = JsonSerializer.Serialize(new { error_code = this.ErrorCode, base64 = IMAGE_BASE64, mime_type = this.MimeType, width = 1, height = 1, was_resized = false });
+        return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent(answer, Encoding.UTF8, "application/json") };
+    }
+}