Fix a couple new tests

Author: ericstj

tests/ModelContextProtocol.Tests/Protocol/PromptMessageTests.cs

@@ -29,7 +29,7 @@ public static void PromptMessage_SerializationRoundTrip_WithImageContent()
         var original = new PromptMessage
         {
             Role = Role.Assistant,
-            Content = new ImageContentBlock { Data = "base64data", MimeType = "image/png" }
+            Content = new ImageContentBlock { Data = System.Text.Encoding.UTF8.GetBytes("base64data"), MimeType = "image/png" }
         };
 
         string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
@@ -38,7 +38,7 @@ public static void PromptMessage_SerializationRoundTrip_WithImageContent()
         Assert.NotNull(deserialized);
         Assert.Equal(Role.Assistant, deserialized.Role);
         var imageBlock = Assert.IsType<ImageContentBlock>(deserialized.Content);
-        Assert.Equal("base64data", imageBlock.Data);
+        Assert.Equal("base64data", System.Text.Encoding.UTF8.GetString(imageBlock.Data.ToArray()));
         Assert.Equal("image/png", imageBlock.MimeType);
     }
 }

tests/ModelContextProtocol.Tests/Protocol/ReadResourceResultTests.cs

@@ -23,7 +23,7 @@ public static void ReadResourceResult_SerializationRoundTrip_PreservesAllPropert
                 {
                     Uri = "file:///image.png",
                     MimeType = "image/png",
-                    Blob = "base64data"
+                    Blob = System.Text.Encoding.UTF8.GetBytes("base64data")
                 }
             ],
             Meta = new JsonObject { ["key"] = "value" }
@@ -43,7 +43,7 @@ public static void ReadResourceResult_SerializationRoundTrip_PreservesAllPropert
         var blobContent = Assert.IsType<BlobResourceContents>(deserialized.Contents[1]);
         Assert.Equal("file:///image.png", blobContent.Uri);
         Assert.Equal("image/png", blobContent.MimeType);
-        Assert.Equal("base64data", blobContent.Blob);
+        Assert.Equal("base64data", System.Text.Encoding.UTF8.GetString(blobContent.Blob.ToArray()));
 
         Assert.NotNull(deserialized.Meta);
         Assert.Equal("value", (string)deserialized.Meta["key"]!);