refactor(tests): centralize fake database import-state cleanup

Test-only refactor continuing #2523. Centralizes the repeated guarded fake core.database/src.database import-state cleanup into a focused helper.

tests/test_topic_analyzer.py

@@ -1,24 +1,12 @@
 """Tests for topic keyword matching (src/topic_analyzer.py)."""
-import sys
 from types import SimpleNamespace
 import pytest
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
 
+from tests.helpers.import_state import clear_fake_database_modules
 
-def _drop_fake_core_database():
-    parent = sys.modules.get("core")
-    attr = getattr(parent, "database", None) if parent is not None else None
-    mod = sys.modules.get("core.database") or attr
-    if mod is None or isinstance(getattr(mod, "__file__", None), str):
-        return
-    sys.modules.pop("core.database", None)
-    sys.modules.pop("src.database", None)
-    if parent is not None and attr is mod:
-        delattr(parent, "database")
-
-
-_drop_fake_core_database()
+clear_fake_database_modules()
 
 from core.database import Base, Session as DbSession, ChatMessage as DbChatMessage
 from core.session_manager import SessionManager