fix: isolate WEB_UI_HOST env var in test and handle empty string

- Patch WEB_UI_HOST in test_web_defaults to avoid env leak
- Handle empty WEB_UI_HOST string in config (fall back to 127.0.0.1)

tests/gateway/test_web.py

@@ -81,6 +81,7 @@ class TestConfigEnvOverrides(unittest.TestCase):
     @patch.dict(os.environ, {
         "WEB_UI_ENABLED": "true",
         "WEB_UI_TOKEN": "",
+        "WEB_UI_HOST": "",
     }, clear=False)
     def test_web_defaults(self):
         config = GatewayConfig()