test: stop test_skill_index_prompt_injection leaking a stub prefs_routes (#4387)

_patch_prefs installs a fake routes.prefs_routes with a bare
sys.modules[...] = assignment that is never undone. The stub is an empty
ModuleType without _save_for_user, so a later test whose code path runs
`from routes.prefs_routes import _save_for_user` (e.g. test_backup_import_skills)
fails with ImportError under an unfavorable test order.

Install the stub with monkeypatch.setitem instead (the helper already takes
monkeypatch and uses it for DATA_DIR) so it is reverted at teardown.

Repro: pytest tests/test_skill_index_prompt_injection.py tests/test_backup_import_skills.py
(1 failed before, 5 passed after).

tests/test_skill_index_prompt_injection.py

@@ -105,7 +105,7 @@ def _patch_prefs(monkeypatch, data_dir):
         "skills_enabled": True,
         "auto_approve_skills": True,
     }
-    sys.modules["routes.prefs_routes"] = fake_prefs
+    monkeypatch.setitem(sys.modules, "routes.prefs_routes", fake_prefs)
 
     # Bust the base-prompt cache so our test re-reads the skill index.
     from src import agent_loop