feat(chat): Add context attachment and knowledge base support

lib/features/chat/models/chat_context_attachment.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/foundation.dart';
+
+/// Represents a non-file attachment that enriches a chat message,
+/// such as a web page, YouTube video transcript, or an existing
+/// knowledge base document reference.
+@immutable
+class ChatContextAttachment {
+  const ChatContextAttachment({
+    required this.id,
+    required this.type,
+    required this.displayName,
+    this.url,
+    this.content,
+    this.collectionName,
+    this.fileId,
+  });
+
+  final String id;
+  final ChatContextAttachmentType type;
+  final String displayName;
+  final String? url;
+  final String? content;
+  final String? collectionName;
+  final String? fileId;
+}
+
+enum ChatContextAttachmentType { web, youtube, knowledge }