refactor(api): 统一响应解析并修复下载错误文案空白

各 *Api 类此前各自实现 data 结构判断，download_api 在结构异常时抛出空
message，界面会显示空白提示。抽出 ResponseDecoders 提供对象、可空对象、
对象数组三种语义，七个文件改为复用；下载侧异常文案与其他接口对齐。

test/core/response_decoders_test.dart

@@ -0,0 +1,87 @@
+// Author: fruit.com.cn
+
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fruitshorts/core/data/remote/api_client.dart';
+import 'package:fruitshorts/core/data/remote/download_api.dart';
+import 'package:fruitshorts/core/data/remote/response_decoders.dart';
+
+import '../support/fake_http_adapter.dart';
+
+void main() {
+  group('ResponseDecoders', () {
+    test('should_return_same_map_when_data_is_object', () {
+      const source = {'id': '1'};
+      expect(ResponseDecoders.asMap(source), same(source));
+    });
+
+    test('should_throw_with_readable_message_when_data_is_not_object', () {
+      expect(
+        () => ResponseDecoders.asMap(const []),
+        throwsA(
+          isA<ApiException>()
+              .having((e) => e.code, 'code', ResponseDecoders.malformedCode)
+              .having(
+                (e) => e.message,
+                'message',
+                ResponseDecoders.malformedMessage,
+              ),
+        ),
+      );
+      expect(ResponseDecoders.malformedMessage, isNotEmpty);
+    });
+
+    test('should_return_null_when_try_map_receives_non_object', () {
+      expect(ResponseDecoders.tryMap(null), isNull);
+      expect(ResponseDecoders.tryMap('text'), isNull);
+      expect(ResponseDecoders.tryMap({'a': 1})?['a'], 1);
+    });
+
+    test('should_skip_non_object_items_when_decoding_object_list', () {
+      final decoded = ResponseDecoders.asMapList([
+        {'id': '1'},
+        'bad',
+        {'id': '2'},
+      ]);
+      expect(decoded.map((item) => item['id']), ['1', '2']);
+    });
+
+    test('should_return_empty_list_when_data_is_not_list', () {
+      expect(ResponseDecoders.asMapList(const {}), isEmpty);
+      expect(ResponseDecoders.asMapList(null), isEmpty);
+    });
+
+    test('should_throw_when_required_list_receives_non_list', () {
+      expect(
+        () => ResponseDecoders.requiredMapList(const {}),
+        throwsA(isA<ApiException>()),
+      );
+    });
+  });
+
+  test(
+    'should_raise_readable_error_when_download_policy_data_is_malformed',
+    () async {
+      final adapter = FakeHttpAdapter(
+        (options) async => FakeHttpResponse(
+          jsonEncode({'code': 0, 'message': 'ok', 'data': null}),
+        ),
+      );
+      final client = ApiClient(adapter: adapter);
+      final api = DownloadApi(client);
+
+      await expectLater(
+        api.policy(),
+        throwsA(
+          isA<ApiException>().having(
+            (error) => error.message,
+            'message',
+            ResponseDecoders.malformedMessage,
+          ),
+        ),
+      );
+      client.close();
+    },
+  );
+}