Use reader.GetBytesFromBase64() with FromBytes() for escaped base64 data

When JSON-escaped base64 data is present, use reader.GetBytesFromBase64()
to directly decode to bytes instead of going through an intermediate
string with GetString() + Encoding.UTF8.GetBytes(). Then use FromBytes()
factory methods which cache the decoded data and lazily encode to base64
UTF-8, avoiding unnecessary intermediate allocations.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/ContentBlock.cs

@@ -4,7 +4,6 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.InteropServices;
-using System.Text;
 using System.Text.Json;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
@@ -93,6 +92,7 @@ public sealed class Converter : JsonConverter<ContentBlock>
             string? name = null;
             string? title = null;
             ReadOnlyMemory<byte>? data = null;
+            ReadOnlyMemory<byte>? decodedData = null;
             string? mimeType = null;
             string? uri = null;
             string? description = null;
@@ -144,7 +144,7 @@ public sealed class Converter : JsonConverter<ContentBlock>
                         }
                         else
                         {
-                            data = Encoding.UTF8.GetBytes(reader.GetString()!);
+                            decodedData = reader.GetBytesFromBase64();
                         }
                         break;
 
@@ -238,17 +238,23 @@ public sealed class Converter : JsonConverter<ContentBlock>
                     Text = text ?? throw new JsonException("Text contents must be provided for 'text' type."),
                 },
 
-                "image" => new ImageContentBlock
-                {
-                    Data = data ?? throw new JsonException("Image data must be provided for 'image' type."),
-                    MimeType = mimeType ?? throw new JsonException("MIME type must be provided for 'image' type."),
-                },
-
-                "audio" => new AudioContentBlock
-                {
-                    Data = data ?? throw new JsonException("Audio data must be provided for 'audio' type."),
-                    MimeType = mimeType ?? throw new JsonException("MIME type must be provided for 'audio' type."),
-                },
+                "image" => decodedData is not null ?
+                    ImageContentBlock.FromBytes(decodedData.Value,
+                        mimeType ?? throw new JsonException("MIME type must be provided for 'image' type.")) :
+                    new ImageContentBlock
+                    {
+                        Data = data ?? throw new JsonException("Image data must be provided for 'image' type."),
+                        MimeType = mimeType ?? throw new JsonException("MIME type must be provided for 'image' type."),
+                    },
+
+                "audio" => decodedData is not null ?
+                    AudioContentBlock.FromBytes(decodedData.Value,
+                        mimeType ?? throw new JsonException("MIME type must be provided for 'audio' type.")) :
+                    new AudioContentBlock
+                    {
+                        Data = data ?? throw new JsonException("Audio data must be provided for 'audio' type."),
+                        MimeType = mimeType ?? throw new JsonException("MIME type must be provided for 'audio' type."),
+                    },
 
                 "resource" => new EmbeddedResourceBlock
                 {

src/ModelContextProtocol.Core/Protocol/ResourceContents.cs

@@ -2,7 +2,6 @@
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
-using System.Text;
 using System.Text.Json;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
@@ -81,6 +80,7 @@ public sealed class Converter : JsonConverter<ResourceContents>
             string? uri = null;
             string? mimeType = null;
             ReadOnlyMemory<byte>? blob = null;
+            ReadOnlyMemory<byte>? decodedBlob = null;
             string? text = null;
             JsonObject? meta = null;
 
@@ -112,7 +112,7 @@ public sealed class Converter : JsonConverter<ResourceContents>
                         }
                         else
                         {
-                            blob = Encoding.UTF8.GetBytes(reader.GetString()!);
+                            decodedBlob = reader.GetBytesFromBase64();
                         }
                         break;
 
@@ -130,6 +130,13 @@ public sealed class Converter : JsonConverter<ResourceContents>
                 }
             }
 
+            if (decodedBlob is not null)
+            {
+                var blobResource = BlobResourceContents.FromBytes(decodedBlob.Value, uri ?? string.Empty, mimeType);
+                blobResource.Meta = meta;
+                return blobResource;
+            }
+
             if (blob is not null)
             {
                 return new BlobResourceContents