.NET: Update A2A, MCP, and system package dependencies (microsoft#4647)

* .NET: Update A2A, MCP, and system package dependencies

Update dependency versions:
- A2A/A2A.AspNetCore: 0.3.3-preview → 0.3.4-preview
- ModelContextProtocol: 0.8.0-preview.1 → 1.1.0
- Microsoft.Bcl.AsyncInterfaces: 10.0.3 → 10.0.4
- System.Linq.AsyncEnumerable: 10.0.0 → 10.0.4
- Add Microsoft.Bcl.Memory 10.0.4

Remove internal polyfill extensions now provided by A2A SDK 0.3.4:
- A2AMetadataExtensions (source + tests)
- AdditionalPropertiesDictionaryExtensions (source + tests)

Update DefaultMcpToolHandler to match MCP SDK 1.1.0 API changes where
ImageContentBlock.Data and AudioContentBlock.Data changed from string
to ReadOnlyMemory<byte>.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* address pr review comments

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SergeyMenshykh

dotnet/Directory.Packages.props

@@ -33,14 +33,15 @@
     <!-- Newtonsoft.Json -->
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.4" />
     <!-- System.* -->
-    <PackageVersion Include="Microsoft.Bcl.AsyncInterfaces" Version="10.0.3" />
+    <PackageVersion Include="Microsoft.Bcl.AsyncInterfaces" Version="10.0.4" />
     <PackageVersion Include="Microsoft.Bcl.HashCode" Version="6.0.0" />
+    <PackageVersion Include="Microsoft.Bcl.Memory" Version="10.0.4" />
     <PackageVersion Include="System.ClientModel" Version="1.9.0" />
     <PackageVersion Include="System.CodeDom" Version="10.0.0" />
     <PackageVersion Include="System.Collections.Immutable" Version="10.0.1" />
     <PackageVersion Include="System.CommandLine" Version="2.0.0-rc.2.25502.107" />
     <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="10.0.3" />
-    <PackageVersion Include="System.Linq.AsyncEnumerable" Version="10.0.0" />
+    <PackageVersion Include="System.Linq.AsyncEnumerable" Version="10.0.4" />
     <PackageVersion Include="System.Net.Http.Json" Version="10.0.0" />
     <PackageVersion Include="System.Net.ServerSentEvents" Version="10.0.3" />
     <PackageVersion Include="System.Text.Json" Version="10.0.3" />
@@ -101,10 +102,10 @@
     <PackageVersion Include="Microsoft.Agents.Authentication.Msal" Version="1.3.171-beta" />
     <PackageVersion Include="Microsoft.Agents.Hosting.AspNetCore" Version="1.3.171-beta" />
     <!-- A2A -->
-    <PackageVersion Include="A2A" Version="0.3.3-preview" />
-    <PackageVersion Include="A2A.AspNetCore" Version="0.3.3-preview" />
+    <PackageVersion Include="A2A" Version="0.3.4-preview" />
+    <PackageVersion Include="A2A.AspNetCore" Version="0.3.4-preview" />
     <!-- MCP -->
-    <PackageVersion Include="ModelContextProtocol" Version="0.8.0-preview.1" />
+    <PackageVersion Include="ModelContextProtocol" Version="1.1.0" />
     <!-- Inference SDKs -->
     <PackageVersion Include="AWSSDK.Extensions.Bedrock.MEAI" Version="4.0.5.1" />
     <PackageVersion Include="Microsoft.ML.OnnxRuntimeGenAI" Version="0.10.0" />

dotnet/src/Microsoft.Agents.AI.A2A/Extensions/A2AMetadataExtensions.cs

@@ -5,6 +5,7 @@
 using System.Globalization;
 using System.Linq;
 using System.Net.Http;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
@@ -222,31 +223,36 @@ private static void PopulateResultContent(McpServerToolResultContent resultConte
         }
     }
 
-    private static AIContent ConvertContentBlock(ContentBlock block)
+    internal static AIContent ConvertContentBlock(ContentBlock block)
     {
         return block switch
         {
             TextContentBlock text => new TextContent(text.Text),
-            ImageContentBlock image => CreateDataContentFromBase64(image.Data, image.MimeType ?? "image/*"),
-            AudioContentBlock audio => CreateDataContentFromBase64(audio.Data, audio.MimeType ?? "audio/*"),
+            ImageContentBlock image => CreateDataContent(image.Data, image.MimeType ?? "image/*"),
+            AudioContentBlock audio => CreateDataContent(audio.Data, audio.MimeType ?? "audio/*"),
             _ => new TextContent(block.ToString() ?? string.Empty),
         };
     }
 
-    private static DataContent CreateDataContentFromBase64(string? base64Data, string mediaType)
+    private static DataContent CreateDataContent(ReadOnlyMemory<byte> base64Utf8Data, string mediaType)
     {
-        if (string.IsNullOrEmpty(base64Data))
+        if (base64Utf8Data.IsEmpty)
         {
             return new DataContent($"data:{mediaType};base64,", mediaType);
         }
 
+#if NET8_0_OR_GREATER
+        string base64 = Encoding.UTF8.GetString(base64Utf8Data.Span);
+#else
+        string base64 = Encoding.UTF8.GetString(base64Utf8Data.ToArray());
+#endif
+
         // If it's already a data URI, use it directly
-        if (base64Data.StartsWith("data:", StringComparison.OrdinalIgnoreCase))
+        if (base64.StartsWith("data:", StringComparison.OrdinalIgnoreCase))
         {
-            return new DataContent(base64Data, mediaType);
+            return new DataContent(base64, mediaType);
         }
 
-        // Otherwise, construct a data URI from the base64 data
-        return new DataContent($"data:{mediaType};base64,{base64Data}", mediaType);
+        return new DataContent($"data:{mediaType};base64,{base64}", mediaType);
     }
 }