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/test/java/tech/easyflow/publicapi/controller/PublicWorkflowControllerContractTest.java

@@ -0,0 +1,26 @@
+package tech.easyflow.publicapi.controller;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.springframework.http.MediaType;
+import org.springframework.web.bind.annotation.RequestMapping;
+
+/**
+ * {@link PublicWorkflowController} HTTP 响应契约测试。
+ */
+public class PublicWorkflowControllerContractTest {
+
+    /**
+     * 验证工作流公共 API 明确返回 JSON，避免通配 Accept 请求被 CBOR 转换器接管。
+     */
+    @Test
+    public void shouldDeclareJsonResponseMediaType() {
+        RequestMapping requestMapping = PublicWorkflowController.class.getAnnotation(RequestMapping.class);
+
+        Assert.assertNotNull(requestMapping);
+        Assert.assertArrayEquals(
+                new String[]{MediaType.APPLICATION_JSON_VALUE},
+                requestMapping.produces()
+        );
+    }
+}