Stop the update-contract test stubs from leaking into the shared suite

test_update_contract installs stub auth/loggers/utils modules into
sys.modules at collection time so routes/llama.py loads standalone, but it
never removed them. In the one-process backend suite that poisoned every
later test: a bare auth package broke 'from auth import storage' and
'from auth.authentication import create_access_token', and the NopLogger
stub (no error method) broke code paths that log errors.

Snapshot the affected sys.modules entries before installing the stubs and
restore them right after the route module has bound its imports; the loaded
route keeps the stub references it captured, so the contract tests are
unchanged. Add a regression test asserting no installed stub is left in
sys.modules. Mirrors the pop-after-load hygiene already in test_llama_route.

studio/backend/tests/test_update_contract.py

@@ -30,6 +30,19 @@ pytest.importorskip("fastapi")
 _HERE = Path(__file__).resolve().parent
 _BACKEND = _HERE.parent
 
+# Stub modules _install_stubs writes into the shared sys.modules, tracked so they
+# are restored after the route loads and a regression test can assert none leak
+# into the rest of the one-process backend suite.
+_INSTALLED_STUBS: dict[str, types.ModuleType] = {}
+_STUBBED_KEYS = (
+    "auth",
+    "auth.authentication",
+    "loggers",
+    "utils",
+    "utils.llama_cpp_update",
+    "utils.update_confirm",
+)
+
 
 def _install_stubs():
     """Stub packages so routes/llama.py imports cleanly, plus the real update_confirm."""
@@ -67,6 +80,7 @@ def _install_stubs():
         utils_pkg = types.ModuleType("utils")
         utils_pkg.__path__ = []
         sys.modules["utils"] = utils_pkg
+        _INSTALLED_STUBS["utils"] = utils_pkg
 
     lcu_mod = types.ModuleType("utils.llama_cpp_update")
 
@@ -105,10 +119,16 @@ def _install_stubs():
         ("utils.llama_cpp_update", lcu_mod),
         ("utils.update_confirm", uc_mod),
     ):
+        _INSTALLED_STUBS[name] = mod
         sys.modules[name] = mod
     return uc_mod
 
 
+# Snapshot the real modules before installing the collection-time stubs, then
+# restore them once the route has bound its stubbed imports. Leaving the stubs in
+# sys.modules poisons the shared process: a bare ``auth`` package and a NopLogger
+# break every later test that imports the real auth/loggers/utils modules.
+_saved_modules = {name: sys.modules.get(name) for name in _STUBBED_KEYS}
 _uc = _install_stubs()
 
 
@@ -124,6 +144,13 @@ def _load_route():
 
 rl = _load_route()
 
+# rl now holds its stub references; put the real modules back so nothing leaks.
+for _name, _orig in _saved_modules.items():
+    if _orig is None:
+        sys.modules.pop(_name, None)
+    else:
+        sys.modules[_name] = _orig
+
 
 @pytest.fixture(autouse = True)
 def _reset(monkeypatch):
@@ -489,3 +516,12 @@ def test_http_local_and_remote_behave_identically(monkeypatch):
     remote = client.post("/api/llama/update", headers = remote_h, json = {"confirmed": True})
     assert local.json()["started"] == remote.json()["started"] == True
     assert calls["n"] == 2  # both proceeded; location was never the gate
+
+
+def test_collection_stubs_do_not_leak_into_sys_modules():
+    # Regression: the collection-time stubs (bare auth package, NopLogger loggers,
+    # default update fns) must be removed from sys.modules after the route loads.
+    # Leaving them poisons the one-process backend suite -- every later test that
+    # imports the real auth/loggers/utils modules fails with "unknown location".
+    for name, stub in _INSTALLED_STUBS.items():
+        assert sys.modules.get(name) is not stub, f"{name} stub leaked into sys.modules"