fix(endpoint): import ModelEndpoint from core database

ModelEndpoint is defined in core.database, not src.database. The wrong
import silently prevented the module from loading in deployment
configurations that do not have a src/database.py shim, resulting in an
ImportError at startup.

Also adds a warning log when resolve_endpoint finds no usable model
(all models hidden or the list is empty), making the otherwise-silent
failure visible in operator logs.

The test_auth_regressions stub for src.endpoint_resolver was missing the
build_models_url attribute, which caused test collection errors.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_auth_regressions.py

@@ -81,13 +81,13 @@ def _auth_regressions_stubs(monkeypatch):
         resolve_endpoint=MagicMock(return_value=("", "", {})),
         normalize_base=MagicMock(),
         build_chat_url=MagicMock(),
+        build_models_url=MagicMock(),
         build_headers=MagicMock(),
     )
     monkeypatch.setitem(sys.modules, "core.database", db)
     monkeypatch.setitem(sys.modules, "core.auth", auth)
     monkeypatch.setitem(sys.modules, "src.endpoint_resolver", ep)
 
-
 from fastapi import HTTPException
 
 # ---------------------------------------------------------------------------