fix: 固定公共工作流接口响应类型

- 统一声明 application/json 响应并补充契约测试

easyflow-api/easyflow-api-public/src/main/java/tech/easyflow/publicapi/controller/PublicWorkflowController.java

@@ -6,6 +6,7 @@ import com.easyagents.flow.core.chain.runtime.ChainExecutor;
 import jakarta.annotation.Resource;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.validation.constraints.NotBlank;
+import org.springframework.http.MediaType;
 import org.springframework.web.bind.annotation.*;
 import tech.easyflow.approval.annotation.RequirePublishedAccess;
 import tech.easyflow.ai.easyagentsflow.entity.ChainInfo;
@@ -38,7 +39,7 @@ import java.util.Map;
 /**
  * 工作流
  */
-@RequestMapping("/public-api/workflow")
+@RequestMapping(value = "/public-api/workflow", produces = MediaType.APPLICATION_JSON_VALUE)
 @RestController
 public class PublicWorkflowController {