refactor(paging): 抽取分页控制器基类收敛四个列表页

收藏、观看记录、剧单、公告各自手写 loading 互斥、请求序号、跨页去重与
错误映射，修一处要同步改四份。统一到 PagedListController 后子类只提供
取页与条目标识；空页按已加载数收口，避免 total 虚大时反复请求。

test/core/paged_list_controller_test.dart

@@ -0,0 +1,129 @@
+// Author: fruit.com.cn
+
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fruitshorts/core/data/models/page_result.dart';
+import 'package:fruitshorts/core/data/remote/api_client.dart';
+import 'package:fruitshorts/core/data/services/paged_list_controller.dart';
+
+/// 用最薄的子类验证基类契约：分页、去重、空页收口与错误映射。
+class _NumberController extends PagedListController<int> {
+  _NumberController(this.pages, {super.fallbackErrorKey});
+
+  final List<PageResult<int>> pages;
+  final List<int> requested = [];
+  Object? failure;
+
+  @override
+  Future<PageResult<int>> fetchPage(int pageIndex) async {
+    requested.add(pageIndex);
+    if (failure != null) throw failure!;
+    if (pages.isEmpty) return const PageResult();
+    return pages.removeAt(0);
+  }
+
+  @override
+  String itemKey(int item) => '$item';
+
+  /// 基类的失效入口是 protected，测试里包一层。
+  void invalidate() => invalidateRequests();
+}
+
+void main() {
+  test('should_append_only_new_items_when_next_page_overlaps', () async {
+    final controller = _NumberController([
+      const PageResult(total: 4, list: [1, 2]),
+      const PageResult(total: 4, list: [2, 3]),
+    ]);
+
+    await controller.load();
+    await controller.loadMore();
+
+    expect(controller.items, [1, 2, 3]);
+    expect(controller.requested, [1, 2]);
+    controller.dispose();
+  });
+
+  test('should_stop_paging_when_empty_page_arrives', () async {
+    final controller = _NumberController([
+      const PageResult(total: 99, list: [1]),
+      const PageResult(total: 99, list: []),
+    ]);
+
+    await controller.load();
+    expect(controller.hasMore, isTrue);
+
+    await controller.loadMore();
+    expect(controller.hasMore, isFalse);
+    await controller.loadMore();
+    expect(controller.requested, [1, 2]);
+    controller.dispose();
+  });
+
+  test('should_expose_backend_message_when_api_fails', () async {
+    final controller = _NumberController([])
+      ..failure = const ApiException(500, '服务暂不可用');
+
+    await controller.load();
+
+    expect(controller.error, '服务暂不可用');
+    expect(controller.lastErrorCode, 500);
+    controller.dispose();
+  });
+
+  test('should_fall_back_to_key_when_failure_is_not_api_error', () async {
+    final controller = _NumberController([], fallbackErrorKey: 'loadFailed')
+      ..failure = StateError('boom');
+
+    await controller.load();
+
+    expect(controller.error, 'loadFailed');
+    controller.dispose();
+  });
+
+  test('should_ignore_stale_response_when_invalidated', () async {
+    final controller = _NumberController([
+      const PageResult(total: 1, list: [1]),
+    ]);
+
+    final pending = controller.load();
+    controller.invalidate();
+    await pending;
+
+    expect(controller.items, isEmpty);
+    expect(controller.loaded, isFalse);
+    controller.dispose();
+  });
+
+  test('should_not_stick_loading_when_refresh_supersedes_load', () async {
+    final first = Completer<PageResult<int>>();
+    final controller = _DeferredController([
+      () => first.future,
+      () async => const PageResult(total: 1, list: [2]),
+    ]);
+
+    final loading = controller.load();
+    final refreshing = controller.refresh();
+    first.complete(const PageResult(total: 1, list: [1]));
+    await Future.wait([loading, refreshing]);
+
+    expect(controller.loading, isFalse);
+    expect(controller.refreshing, isFalse);
+    expect(controller.items, [2]);
+    controller.dispose();
+  });
+}
+
+/// 让每个请求各自挂起，用来构造「后发请求抢占先发请求」的时序。
+class _DeferredController extends PagedListController<int> {
+  _DeferredController(this._pages);
+
+  final List<Future<PageResult<int>> Function()> _pages;
+
+  @override
+  Future<PageResult<int>> fetchPage(int pageIndex) => _pages.removeAt(0)();
+
+  @override
+  String itemKey(int item) => '$item';
+}

test/core/playlist_controller_test.dart

@@ -22,12 +22,25 @@ void main() {
 
     controller.dispose();
   });
+
+  test('should_keep_duplicate_ids_when_repository_repeats_items', () async {
+    // 剧单不去重：服务端分页错位时按原样展示，不能悄悄吞掉条目。
+    final repository = _PagedPlaylists(total: 12, repeatLastOfFirstPage: true);
+    final controller = PlaylistController(repository);
+
+    await controller.load();
+    await controller.loadMore();
+
+    expect(controller.items, hasLength(12));
+    controller.dispose();
+  });
 }
 
 class _PagedPlaylists implements PlaylistRepository {
-  _PagedPlaylists({required this.total});
+  _PagedPlaylists({required this.total, this.repeatLastOfFirstPage = false});
 
   final int total;
+  final bool repeatLastOfFirstPage;
 
   @override
   Future<PageResult<PlaylistDetail>> fetchPlaylists({
@@ -44,7 +57,7 @@ class _PagedPlaylists implements PlaylistRepository {
       list: [
         for (var index = start; index < end; index++)
           PlaylistDetail(
-            id: '$index',
+            id: '${repeatLastOfFirstPage && index == pageSize ? start - 1 : index}',
             name: '剧单$index',
             description: '',
             dramaCount: 0,