feat(chat): regenerate variants and support

Hide archived assistant variants in the linear chat view and track
previous assistant as versions so regenerated responses do not
duplicate or lose history. When regenerating, mark the previous assistant
message with an archivedVariant flag for the UI and keep it in server
history. Add a ChatMessageVersion model and a versions field to
ChatMessage to store prior generated variants. Implement
archiveLastAssistantAsVersion in chat providers to snapshot the last
assistant message into versions and reset the message for a fresh
streamed generation. Finalize flow updates to attach an adjacent archived
assistant as a version when needed so the UI can present a switcher
between current and past variants. These changes prevent duplicate
messages, preserve previous responses, and enable variant switching.

lib/core/models/chat_message.dart

@@ -30,12 +30,40 @@ sealed class ChatMessage with _$ChatMessage {
     @Default(<ChatSourceReference>[])
     List<ChatSourceReference> sources,
     Map<String, dynamic>? usage,
+    // Previous generated versions of this assistant message (OpenWebUI-style)
+    @JsonKey(includeFromJson: false, includeToJson: false)
+    @Default(<ChatMessageVersion>[])
+    List<ChatMessageVersion> versions,
   }) = _ChatMessage;
 
   factory ChatMessage.fromJson(Map<String, dynamic> json) =>
       _$ChatMessageFromJson(json);
 }
 
+@freezed
+abstract class ChatMessageVersion with _$ChatMessageVersion {
+  const factory ChatMessageVersion({
+    required String id,
+    required String content,
+    required DateTime timestamp,
+    String? model,
+    List<Map<String, dynamic>>? files,
+    @JsonKey(
+      name: 'sources',
+      fromJson: _sourceRefsFromJson,
+      toJson: _sourceRefsToJson,
+    )
+    @Default(<ChatSourceReference>[])
+    List<ChatSourceReference> sources,
+    @Default(<String>[]) List<String> followUps,
+    @Default(<ChatCodeExecution>[]) List<ChatCodeExecution> codeExecutions,
+    Map<String, dynamic>? usage,
+  }) = _ChatMessageVersion;
+
+  factory ChatMessageVersion.fromJson(Map<String, dynamic> json) =>
+      _$ChatMessageVersionFromJson(json);
+}
+
 @freezed
 abstract class ChatStatusUpdate with _$ChatStatusUpdate {
   const factory ChatStatusUpdate({