refactor(tests): add import-state isolation helper

Test-only refactor continuing #2523. Adds a shared import-state isolation helper with focused coverage and migrates two pilot tests that manually preserved sys.modules and parent package attributes.

tests/test_blind_compare_redaction.py

@@ -22,77 +22,23 @@ import importlib
 from pathlib import Path
 from unittest.mock import MagicMock
 
+from tests.helpers.import_state import clear_module, preserve_import_state
+
 _REPO = Path(__file__).resolve().parent.parent
 
-# Mirror tests/test_session_ghost_delete.py exactly: stub only the ORM *class*
-# modules and import the REAL core.session_manager + src.auth_helpers. pytest
-# caches routes.session_routes after the first import, so stubbing auth_helpers /
-# session_manager here would poison the shared module for the sibling session
-# tests (whichever file is collected first wins). Matching their stub set keeps
-# the cached module identical regardless of collection order. We restore both
-# sys.modules AND the parent `routes` package attribute so the stub-bound module
-# never leaks into sibling modules via `import routes.session_routes as X`.
-_ABSENT = object()
-
-
-def _save_module_and_parent_attr(dotted_name):
-    """Capture a module's sys.modules entry *and* its parent-package attribute.
-
-    Importing ``routes.session_routes`` also sets ``session_routes`` on the
-    parent ``routes`` package object, and ``import routes.session_routes as X``
-    resolves ``X`` through that parent attribute — so restoring sys.modules
-    alone leaves the stale stub-bound module reachable. Returns a (module, attr)
-    pair to hand back to _restore_module_and_parent_attr.
-    """
-    saved_module = sys.modules.get(dotted_name, _ABSENT)
-    pkg_name, _, attr = dotted_name.rpartition(".")
-    pkg = sys.modules.get(pkg_name)
-    saved_attr = getattr(pkg, attr, _ABSENT) if pkg is not None else _ABSENT
-    return saved_module, saved_attr
-
-
-def _restore_module_and_parent_attr(dotted_name, saved_module, saved_attr):
-    """Restore (or remove) both the sys.modules entry and the parent attribute.
-
-    Passing _ABSENT for both clears the cache, which is how we drop any stale
-    entry before the stubbed import.
-    """
-    if saved_module is _ABSENT:
-        sys.modules.pop(dotted_name, None)
-    else:
-        sys.modules[dotted_name] = saved_module
-    pkg_name, _, attr = dotted_name.rpartition(".")
-    pkg = sys.modules.get(pkg_name)
-    if pkg is None:
-        return
-    if saved_attr is _ABSENT:
-        if hasattr(pkg, attr):
-            delattr(pkg, attr)
-    else:
-        setattr(pkg, attr, saved_attr)
-
-
+# Stub only the ORM class modules and import the real core.session_manager so
+# the cached routes.session_routes is identical regardless of collection order.
+# preserve_import_state restores both sys.modules and parent-package attributes
+# after the block, preventing stub leakage into siblings.
 _TEMP_STUBS = ("core.database", "core.models")
-_saved = {name: sys.modules.get(name, _ABSENT) for name in _TEMP_STUBS}
-_saved["core.session_manager"] = sys.modules.get("core.session_manager", _ABSENT)
-_sr_saved = _save_module_and_parent_attr("routes.session_routes")
-try:
+with preserve_import_state(*_TEMP_STUBS, "core.session_manager", "routes.session_routes"):
     for _name in _TEMP_STUBS:
         sys.modules[_name] = MagicMock(name=_name)
     if isinstance(sys.modules.get("core.session_manager"), MagicMock):
         del sys.modules["core.session_manager"]
-    # Clear the sys.modules entry AND the parent `routes` attribute so the
-    # stubbed import below produces a fresh module with no stale binding behind it.
-    _restore_module_and_parent_attr("routes.session_routes", _ABSENT, _ABSENT)
+    clear_module("routes.session_routes")
     importlib.import_module("core.session_manager")
     import routes.session_routes as SR  # noqa: E402
-finally:
-    for _name, _val in _saved.items():
-        if _val is _ABSENT:
-            sys.modules.pop(_name, None)
-        else:
-            sys.modules[_name] = _val
-    _restore_module_and_parent_attr("routes.session_routes", *_sr_saved)
 
 
 # ── backend: GET /api/sessions model redaction ─────────────────────────────