feat：实现聊天系统核心功能

- 添加 SocketIOClient.gd 实现 Socket.IO 协议封装
- 添加 WebSocketManager.gd 管理连接生命周期和自动重连
- 添加 ChatManager.gd 实现聊天业务逻辑与会话管理
  - 支持当前会话缓存（最多 100 条消息）
  - 支持历史消息按需加载（每次 100 条）
  - 每次登录/重连自动重置会话缓存
  - 客户端频率限制（10 条/分钟）
  - Token 管理与认证
- 添加 ChatMessage.gd/tscn 消息气泡 UI 组件
- 添加 ChatUI.gd/tscn 聊天界面
- 在 EventNames.gd 添加 7 个聊天事件常量
- 在 AuthManager.gd 添加 game_token 管理方法
- 添加完整的单元测试（128 个测试用例）
  - test_socketio_client.gd (42 个测试)
  - test_websocket_manager.gd (38 个测试)
  - test_chat_manager.gd (48 个测试)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scenes/ui/ChatUI.tscn

@@ -0,0 +1,77 @@
+[gd_scene load_steps=2 format=3 uid="uid://bv7k2m9n4xj8q"]
+
+[ext_resource type="Script" path="res://scenes/ui/ChatUI.gd" id="1"]
+
+[node name="ChatUI" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+script = ExtResource("1")
+
+[node name="ChatPanel" type="Panel" parent="."]
+unique_name_in_owner = true
+layout_mode = 1
+anchors_preset = 3
+anchor_left = 1.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+offset_left = -450.0
+offset_top = -400.0
+grow_horizontal = 0
+grow_vertical = 0
+
+[node name="VBoxContainer" type="VBoxContainer" parent="ChatPanel"]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+theme_override_constants/separation = 8
+offset_left = 10.0
+offset_top = 10.0
+offset_right = -10.0
+offset_bottom = -10.0
+
+[node name="HeaderContainer" type="HBoxContainer" parent="ChatPanel/VBoxContainer"]
+layout_mode = 2
+
+[node name="StatusLabel" type="Label" parent="ChatPanel/VBoxContainer/HeaderContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+theme_override_colors/font_color = Color(0.5, 0.5, 0.5, 1)
+theme_override_font_sizes/font_size = 12
+text = "○ 未连接"
+
+[node name="ChatHistory" type="ScrollContainer" parent="ChatPanel/VBoxContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_vertical = 3
+
+[node name="MessageList" type="VBoxContainer" parent="ChatPanel/VBoxContainer/ChatHistory"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+size_flags_vertical = 3
+theme_override_constants/separation = 8
+
+[node name="InputContainer" type="HBoxContainer" parent="ChatPanel/VBoxContainer"]
+layout_mode = 2
+theme_override_constants/separation = 8
+
+[node name="ChatInput" type="LineEdit" parent="ChatPanel/VBoxContainer/InputContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+placeholder_text = "输入消息..."
+
+[node name="SendButton" type="Button" parent="ChatPanel/VBoxContainer/InputContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "发送"