Fix SSO test flakiness by mocking premium_user correctly

The test_sso_key_generate_shows_deprecation_banner test was failing in CI
with a 403 Forbidden error because the SSO endpoint checks for premium_user
at line 297 in ui_sso.py.

The fix adds a monkeypatch for premium_user at its source location
(litellm.proxy.proxy_server.premium_user) to bypass the enterprise check
during testing.

Fixes the intermittent test failure where the endpoint would return 403
instead of the expected 200 status code.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/test_litellm/proxy/test_proxy_server.py

@@ -262,6 +262,11 @@ def test_sso_key_generate_shows_deprecation_banner(client_no_auth, monkeypatch):
         "litellm.proxy.management_endpoints.ui_sso.SSOAuthenticationHandler.should_use_sso_handler",
         lambda *args, **kwargs: False,
     )
+    # Mock premium_user to bypass enterprise check (prevents 403 Forbidden)
+    monkeypatch.setattr(
+        "litellm.proxy.proxy_server.premium_user",
+        True,
+    )
     monkeypatch.setenv("UI_USERNAME", "admin")
 
     response = client_no_auth.get("/sso/key/generate")