address review: replace assert in _auth with explicit RuntimeError

Assertions can be disabled at runtime, so use an explicit check and
raise instead — matches how driver creation failure is already handled.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

superset/utils/webdriver.py

@@ -561,7 +561,8 @@ class WebDriverSelenium(WebDriverProxy):
 
     def _auth(self, user: User) -> None:
         """Authenticate the persistent driver in-place."""
-        assert self._driver is not None
+        if self._driver is None:
+            raise RuntimeError("WebDriver is not initialized")
         machine_auth_provider_factory.instance.authenticate_webdriver(
             self._driver, user
         )