feat: add clipboard image handling for pasting and staging attachments

src/EchoHub.Tests/ClipboardImageTests.cs

@@ -0,0 +1,78 @@
+using EchoHub.Client.Services;
+using EchoHub.Core.Services;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Formats.Bmp;
+using SixLabors.ImageSharp.PixelFormats;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class ClipboardImageTests
+{
+    /// <summary>
+    /// Encodes an image as BMP and strips the 14-byte BITMAPFILEHEADER, producing exactly what
+    /// the Windows clipboard hands out as CF_DIB.
+    /// </summary>
+    private static byte[] MakeDib(Image<Rgba32> image, BmpBitsPerPixel bpp)
+    {
+        using var ms = new MemoryStream();
+        image.Save(ms, new BmpEncoder { BitsPerPixel = bpp });
+        return ms.ToArray()[14..];
+    }
+
+    private static Image<Rgba32> MakeTestImage()
+    {
+        var image = new Image<Rgba32>(4, 3);
+        image[0, 0] = new Rgba32(255, 0, 0);
+        image[3, 2] = new Rgba32(0, 0, 255);
+        return image;
+    }
+
+    [Theory]
+    [InlineData(BmpBitsPerPixel.Pixel24)]
+    [InlineData(BmpBitsPerPixel.Pixel32)]
+    [InlineData(BmpBitsPerPixel.Pixel8)] // palette-based: exercises the palette offset math
+    public void DibToPng_ConvertsDibToValidPng(BmpBitsPerPixel bpp)
+    {
+        using var original = MakeTestImage();
+        var dib = MakeDib(original, bpp);
+
+        var png = ClipboardImage.DibToPng(dib);
+
+        Assert.NotNull(png);
+        using var pngStream = new MemoryStream(png);
+        Assert.True(FileValidationHelper.IsValidImage(pngStream));
+
+        using var decoded = Image.Load<Rgba32>(png);
+        Assert.Equal(original.Width, decoded.Width);
+        Assert.Equal(original.Height, decoded.Height);
+    }
+
+    [Fact]
+    public void DibToPng_PreservesPixels_For24Bpp()
+    {
+        using var original = MakeTestImage();
+        var dib = MakeDib(original, BmpBitsPerPixel.Pixel24);
+
+        var png = ClipboardImage.DibToPng(dib);
+
+        Assert.NotNull(png);
+        using var decoded = Image.Load<Rgba32>(png);
+        Assert.Equal(new Rgba32(255, 0, 0), decoded[0, 0]);
+        Assert.Equal(new Rgba32(0, 0, 255), decoded[3, 2]);
+    }
+
+    [Fact]
+    public void DibToPng_ReturnsNull_ForTruncatedData()
+    {
+        Assert.Null(ClipboardImage.DibToPng([0x28, 0x00, 0x00]));
+    }
+
+    [Fact]
+    public void DibToPng_ReturnsNull_ForGarbageData()
+    {
+        var garbage = new byte[256];
+        new Random(42).NextBytes(garbage);
+        Assert.Null(ClipboardImage.DibToPng(garbage));
+    }
+}