fix(tests): 补回 add-plugin-modal.spec 缺失的 builtinTabPlugins import

#535（i18n catalog names）给 tests/add-plugin-modal.spec.tsx 新增
sentinelEntry 行时漏了 import；#535 自身 CI run 被 cancel，#536 带病
合入，main HEAD 7b5c7dd 的 typecheck/test 双红（本地 stash 复现一致：
TS2304 / ReferenceError: builtinTabPlugins is not defined，CI run
33754439466 failure）。补上与 plugin-list.spec.ts 同源的 import。

与本 PR 主题无关，仅为本流验证门（typecheck/test 全绿）所需的最小
main 修复，独立成 commit 便于 review 剔除。

tests/add-plugin-modal.spec.tsx

@@ -19,6 +19,7 @@ import { createRoot } from 'react-dom/client'
 import { act } from 'react-dom/test-utils'
 import * as primitives from '@deepseek-ai/dsh-client-ui-primitives'
 import { type PluginEntry } from '../src/client/plugins-shared.ts'
+import { builtinTabPlugins } from '../src/client/plugins-tabs.ts'
 import { builtinViewerPlugins } from '../src/client/plugins-viewers.ts'
 import { AddPluginModal, PluginListBody } from '../src/client/add-plugin-modal.tsx'
 import { createBetterSidebarService } from '../src/client/service.ts'