fix(ci): 增强 Windows 构建中的 build_runner 错误检查

- 将 build_runner 步骤独立出来
- 添加 PowerShell 文件存在性检查
- 如果生成的文件不存在则立即失败
- 提供详细的诊断日志

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

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

.github/workflows/build-multiplatform.yml

@@ -391,8 +391,30 @@ jobs:
       - name: 📦 安装 Flutter 依赖
         run: |
           flutter pub get
+
+      - name: 🔧 生成代码文件 (build_runner)
+        run: |
+          echo "🔧 开始运行 build_runner..."
           flutter pub run build_runner build --delete-conflicting-outputs
 
+          echo ""
+          echo "✅ build_runner 完成，检查生成的文件..."
+
+          if (Test-Path "lib\singbox\model\singbox_status.freezed.dart") {
+            echo "✅ singbox_status.freezed.dart 已生成"
+          } else {
+            echo "❌ singbox_status.freezed.dart 未生成"
+            exit 1
+          }
+
+          if (Test-Path "lib\singbox\service\singbox_service_provider.g.dart") {
+            echo "✅ singbox_service_provider.g.dart 已生成"
+          } else {
+            echo "❌ singbox_service_provider.g.dart 未生成"
+            exit 1
+          }
+        shell: pwsh
+
       - name: 🔨 构建 Windows (Release)
         run: |
           flutter build windows --release