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.TestSseServer/Program.cs

@@ -85,12 +85,7 @@ public class Program
                     Name = $"Resource {i + 1}",
                     MimeType = "application/octet-stream"
                 });
-                resourceContents.Add(new BlobResourceContents
-                {
-                    Uri = uri,
-                    MimeType = "application/octet-stream",
-                    Blob = Convert.ToBase64String(buffer)
-                });
+                resourceContents.Add(BlobResourceContents.FromBytes(buffer, uri, "application/octet-stream"));
             }
         }
 
@@ -381,7 +376,7 @@ public class Program
                         Role = Role.User,
                         Content = new ImageContentBlock
                         {
-                            Data = MCP_TINY_IMAGE,
+                            Data = System.Text.Encoding.UTF8.GetBytes(MCP_TINY_IMAGE),
                             MimeType = "image/png"
                         }
                     });