✨ feat(sync): 新增 SQL 结果集数据同步能力

- 同步引擎新增查询结果集同步分支，支持单目标表差异分析、预览与执行
- 数据同步工作台增加 SQL 结果集模式，并补充目标表与查询校验
- 补充后端同步链路与前端请求构造回归测试，并更新 backlog 记录

Fixes #321

frontend/src/components/dataSyncRequest.test.ts

@@ -0,0 +1,67 @@
+import { describe, expect, it } from 'vitest';
+import { buildDataSyncRequest, validateDataSyncSelection } from './dataSyncRequest';
+
+describe('validateDataSyncSelection', () => {
+  it('requires source query and single target table in query mode', () => {
+    expect(validateDataSyncSelection({
+      sourceDatasetMode: 'query',
+      selectedTables: [],
+      sourceQuery: '',
+      syncContent: 'data',
+    })).toBe('请输入源查询 SQL');
+
+    expect(validateDataSyncSelection({
+      sourceDatasetMode: 'query',
+      selectedTables: [],
+      sourceQuery: 'select 1',
+      syncContent: 'data',
+    })).toBe('SQL 结果集同步需要选择一个目标表');
+
+    expect(validateDataSyncSelection({
+      sourceDatasetMode: 'query',
+      selectedTables: ['users', 'orders'],
+      sourceQuery: 'select 1',
+      syncContent: 'data',
+    })).toBe('SQL 结果集同步需要选择一个目标表');
+  });
+
+  it('forces data-only in query mode', () => {
+    expect(validateDataSyncSelection({
+      sourceDatasetMode: 'query',
+      selectedTables: ['users'],
+      sourceQuery: 'select 1',
+      syncContent: 'both',
+    })).toBe('SQL 结果集同步仅支持仅同步数据');
+  });
+});
+
+describe('buildDataSyncRequest', () => {
+  it('normalizes query mode payload for backend', () => {
+    const payload = buildDataSyncRequest({
+      sourceConfig: { type: 'mysql' },
+      targetConfig: { type: 'mysql' },
+      selectedTables: ['users'],
+      sourceDatasetMode: 'query',
+      sourceQuery: '  SELECT id, name FROM active_users  ',
+      syncContent: 'both',
+      syncMode: 'insert_update',
+      autoAddColumns: true,
+      targetTableStrategy: 'smart',
+      createIndexes: true,
+      mongoCollectionName: '  ',
+      jobId: 'job-1',
+      tableOptions: { users: { insert: true, update: true, delete: false } },
+    });
+
+    expect(payload).toMatchObject({
+      tables: ['users'],
+      sourceQuery: 'SELECT id, name FROM active_users',
+      content: 'data',
+      mode: 'insert_update',
+      autoAddColumns: false,
+      targetTableStrategy: 'existing_only',
+      createIndexes: false,
+      jobId: 'job-1',
+    });
+  });
+});