fix: URL-encode OAuth error message in redirect URL (open-webui#22415)

- URL-encodes the OAuth error message when constructing the redirect URL in the OIDC callback handler
- Without encoding, error messages containing spaces, ampersands, or other special characters produce malformed URLs that the frontend cannot parse correctly
- The custom OAuth client callback handler already correctly uses urllib.parse.quote_plus() for the same purpose; this fix brings the OIDC handler in line with that pattern

Co-authored-by: gambletan <tan@gambletan.com>

Author: gambletan

backend/open_webui/utils/oauth.py

@@ -1706,7 +1706,7 @@ async def handle_callback(self, request, provider, response, db=None):
         redirect_url = f"{redirect_base_url}/auth"
 
         if error_message:
-            redirect_url = f"{redirect_url}?error={error_message}"
+            redirect_url = f"{redirect_url}?error={urllib.parse.quote_plus(error_message)}"
             return RedirectResponse(url=redirect_url, headers=response.headers)
 
         response = RedirectResponse(url=redirect_url, headers=response.headers)