Use ReadOnlyMemory<byte> for binary data to eliminate UTF-16 transcoding (#1070)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: ericstj <8918108+ericstj@users.noreply.github.com>
Co-authored-by: Eric StJohn <ericstj@microsoft.com>

tests/ModelContextProtocol.Tests/Protocol/CreateMessageResultTests.cs

@@ -116,11 +116,7 @@ public class CreateMessageResultTests
             Model = "test-model",
             Content =
             [
-                new ImageContentBlock
-                {
-                    Data = Convert.ToBase64String([1, 2, 3, 4, 5]),
-                    MimeType = "image/png"
-                }
+                ImageContentBlock.FromBytes((byte[])[1, 2, 3, 4, 5], "image/png")
             ],
             StopReason = "endTurn"
         };