refactor: reasoning parser

lib/core/utils/message_segments.dart

@@ -0,0 +1,24 @@
+import 'tool_calls_parser.dart';
+import 'reasoning_parser.dart';
+
+/// Unified segment representing ordered pieces of a message:
+/// - `text`: plain text/markdown to render
+/// - `toolCall`: a parsed tool call entry to render as a tile
+/// - `reasoning`: a parsed reasoning entry to render as a tile
+class MessageSegment {
+  final String? text;
+  final ToolCallEntry? toolCall;
+  final ReasoningEntry? reasoning;
+
+  const MessageSegment._({this.text, this.toolCall, this.reasoning});
+
+  factory MessageSegment.text(String text) => MessageSegment._(text: text);
+  factory MessageSegment.tool(ToolCallEntry tool) =>
+      MessageSegment._(toolCall: tool);
+  factory MessageSegment.reason(ReasoningEntry entry) =>
+      MessageSegment._(reasoning: entry);
+
+  bool get isText => text != null;
+  bool get isTool => toolCall != null;
+  bool get isReasoning => reasoning != null;
+}