test: add Dexie CRUD tests for history.ts

[felladrin]

• Resolves Add tests for history.ts (Dexie CRUD) #2168

Root cause

client/modules/history.ts had no test coverage for the Dexie CRUD functions (addSearchToHistory, getRecentSearches, updateSearchResults, saveLlmResponseForQuery, getLatestLlmResponseForEntry, saveChatMessageForQuery, getChatMessagesForQuery). Only search-run-ID helpers and pure helper functions were covered.

What changed

File	Change
client/modules/history.test.ts	Added 16 new tests covering all Dexie CRUD operations using fake-indexeddb in jsdom. Mocked ./pubSub so the cleanup hook does not fail on missing settings.

How it was verified

npx vitest run client/modules/history.test.ts  # 31 tests passed
npx vitest run                                  # 376 tests passed
npm run lint                                    # 0 errors