refactor(tiers): replace importlib.reload with mock.patch in demo_tier tests

- Replace fragile reload pattern with unittest.mock.patch('app.wizard.tiers._DEMO_MODE', ...)
- Eliminates parallel test run failures (pytest-xdist) and improves test isolation
- All 4 demo_tier tests now use context managers for clean setup/teardown
- Add explanatory comment to _DEMO_MODE definition about immutability and env-based init

app/wizard/tiers.py

@@ -81,6 +81,9 @@ BYOK_UNLOCKABLE: frozenset[str] = frozenset({
 
 # Demo mode flag — read from environment at module load time.
 # Allows demo toolbar to override tier without accessing st.session_state (thread-safe).
+# _DEMO_MODE is immutable after import for the process lifetime.
+# DEMO_MODE must be set in the environment before the process starts (e.g., via
+# Docker Compose environment:). Runtime toggling is not supported.
 _DEMO_MODE = _os.environ.get("DEMO_MODE", "").lower() in ("1", "true", "yes")
 
 # Free integrations (not in FEATURES):

tests/test_wizard_tiers.py

@@ -1,5 +1,7 @@
 import sys
 from pathlib import Path
+from unittest.mock import patch
+
 sys.path.insert(0, str(Path(__file__).parent.parent))
 
 from app.wizard.tiers import can_use, tier_label, TIERS, FEATURES, BYOK_UNLOCKABLE
@@ -129,43 +131,24 @@ def test_vue_ui_beta_premium_tier():
 
 
 def test_can_use_demo_tier_overrides_real_tier():
-    # demo_tier kwarg substitutes for the real tier when provided and DEMO_MODE is set
-    import os
-    os.environ["DEMO_MODE"] = "true"
-    # Need to reload the module to pick up the new DEMO_MODE value
-    import importlib
-    import app.wizard.tiers as tiers_module
-    importlib.reload(tiers_module)
-    try:
-        assert tiers_module.can_use("free", "company_research", demo_tier="paid") is True
-    finally:
-        # Cleanup: restore original state
-        os.environ.pop("DEMO_MODE", None)
-        importlib.reload(tiers_module)
+    # demo_tier="paid" overrides real tier "free" when DEMO_MODE is active
+    with patch('app.wizard.tiers._DEMO_MODE', True):
+        assert can_use("free", "company_research", demo_tier="paid") is True
 
 
 def test_can_use_demo_tier_free_restricts():
-    # demo_tier="free" should restrict access even if real tier is "paid"
-    import os
-    os.environ["DEMO_MODE"] = "true"
-    import importlib
-    import app.wizard.tiers as tiers_module
-    importlib.reload(tiers_module)
-    try:
-        assert tiers_module.can_use("paid", "model_fine_tuning", demo_tier="free") is False
-    finally:
-        os.environ.pop("DEMO_MODE", None)
-        importlib.reload(tiers_module)
+    # demo_tier="free" restricts access even if real tier is "paid"
+    with patch('app.wizard.tiers._DEMO_MODE', True):
+        assert can_use("paid", "model_fine_tuning", demo_tier="free") is False
 
 
 def test_can_use_demo_tier_none_falls_back_to_real():
-    # demo_tier=None means no override — real tier is used
-    assert can_use("paid", "company_research", demo_tier=None) is True
+    # demo_tier=None means no override regardless of DEMO_MODE
+    with patch('app.wizard.tiers._DEMO_MODE', True):
+        assert can_use("paid", "company_research", demo_tier=None) is True
 
 
 def test_can_use_demo_tier_does_not_affect_non_demo():
-    # demo_tier is only applied when DEMO_MODE env var is set;
-    # in tests DEMO_MODE is False by default, so demo_tier is ignored
-    import os
-    os.environ.pop("DEMO_MODE", None)
-    assert can_use("free", "company_research", demo_tier="paid") is False
+    # When _DEMO_MODE is False, demo_tier is ignored
+    with patch('app.wizard.tiers._DEMO_MODE', False):
+        assert can_use("free", "company_research", demo_tier="paid") is False