In multi-project mode, all /chat/sessions* handlers used options.chatStore
(the home-dir project's store) regardless of the projectId query param.
Sessions in secondary projects were invisible via the API.
Root cause: registerChatRoutes accessed options.chatStore directly instead
of routing through resolveProjectChatContext (already used correctly in
registerChatRoomRoutes for the rooms API).
Fix: introduce resolveScopedChatStore(projectId) helper that delegates to
resolveProjectChatContext, replacing all ten options.chatStore usages.
Falls back to default store when engineManager is absent (backward compat).
Tests: add two cases to verify engine-scoped chatStore is used when
engineManager is configured for the requested projectId.
7f01b5341c