docs: fix non-compiling attachment examples in the .NET README

The three attachment examples in dotnet/README.md referenced a type family
that no longer exists in the SDK: UserMessageDataAttachmentsItem,
UserMessageDataAttachmentsItemFile, UserMessageDataAttachmentsItemBlob and
the enum UserMessageDataAttachmentsItemType. Copying any of them produced
CS0246/CS0103, and because this README is packed as the package README it is
also what renders on the GitHub.Copilot.SDK NuGet page.

Update the examples to the shipped public API - List<Attachment> holding
AttachmentFile and AttachmentBlob - matching the C# blocks in
docs/features/image-input.md that documentation validation compiles. The
explicit discriminator assignment in the file attachment example is dropped
because the concrete subclass supplies it.

Author: examon

dotnet/README.md

@@ -296,9 +296,9 @@ The SDK supports image attachments via the `Attachments` parameter. You can atta
 await session.SendAsync(new MessageOptions
 {
     Prompt = "What's in this image?",
-    Attachments = new List<UserMessageDataAttachmentsItem>
+    Attachments = new List<Attachment>
     {
-        new UserMessageDataAttachmentsItemFile
+        new AttachmentFile
         {
             Path = "/path/to/image.jpg",
             DisplayName = "image.jpg",
@@ -310,9 +310,9 @@ await session.SendAsync(new MessageOptions
 await session.SendAsync(new MessageOptions
 {
     Prompt = "What's in this image?",
-    Attachments = new List<UserMessageDataAttachmentsItem>
+    Attachments = new List<Attachment>
     {
-        new UserMessageDataAttachmentsItemBlob
+        new AttachmentBlob
         {
             Data = base64ImageData,
             MimeType = "image/png",
@@ -721,11 +721,10 @@ await session2.SendAsync(new MessageOptions { Prompt = "Hello from session 2" })
 await session.SendAsync(new MessageOptions
 {
     Prompt = "Analyze this file",
-    Attachments = new List<UserMessageDataAttachmentsItem>
+    Attachments = new List<Attachment>
     {
-        new UserMessageDataAttachmentsItem
+        new AttachmentFile
         {
-            Type = UserMessageDataAttachmentsItemType.File,
             Path = "/path/to/file.cs",
             DisplayName = "My File"
         }