test(frontend): add unit test coverage for useChatHistory hook #1165

Add renderHook-based integration tests for useChatHistory covering:
- Initial state (empty sessions, null currentSessionId, isHistoryOpen)
- createNewSession (creation, initial messages, session counting)
- updateCurrentSession (message updates, no-op guard, timestamp update)
- loadSession (valid/invalid IDs, currentSessionId switching)
- deleteSession (removal, clearing active, preserving other sessions)
- clearAllHistory (state reset, localStorage removal)
- togglePin (pin/unpin, no-op for invalid IDs)
- Export functions (JSON and TXT, valid and invalid IDs)
- Debounced search (with fake timers, empty/no-match queries)
- Unmount cleanup (save and search debounce timeout clearing)
- sorted sessions (pinnedSessions, unpinnedSessions, currentSession)
- setIsHistoryOpen toggle
- reorderPinned ordering
- Negative tests for every operation with invalid/missing input

Also fixes pre-existing structural issues in the race-condition
describe blocks (missing closing braces, nested describe in wrong
scope).

Author: Yodomite