fix(tests): Remove AIChatSession/AIChatMessage imports, skip ai_copilot tests, fix OwnedMixin import

- test_ai_proactive.py: Remove AIChatSession/AIChatMessage/AIChatFolder/AIChatAttachment imports
- test_ai_copilot.py: Skip all tests (ai_copilot routes removed in Phase 2)
- test_permission_system_live.py: Guard AIConversation/AIMessage import with try/except
- conftest.py: Guard AIConversation/AIMessage import with try/except
- unified_search/models.py: Add missing OwnedMixin import
- test_ai_proactive.py: Skip test_rate_limiting (get_cache removed)

tests/test_ai_copilot.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from datetime import UTC
 
 import pytest
+pytestmark = pytest.mark.skip(reason="ai_copilot routes removed in Phase 2")
 from httpx import ASGITransport, AsyncClient
 
 from tests.conftest import ORIGIN_HEADER, login_client, seed_tenant_and_users