refac: make the web search error message a plain constant (#28948)

The web search error message was a lambda with a passthrough branch that returned whatever it was handed. Since #28942 both call sites pass no arguments, so that branch is unreachable, and it is the trap that let a caller drop a raw exception object into an HTTP response body and turn an intended 400 into an unserialisable 500.

A plain string constant removes the trap and lines the message up with every other fixed message in that file. Behaviour is unchanged: the response detail comes out byte for byte identical, because the enum already overrides __str__ to render members as their value. Verified on Python 3.11 and 3.12, both producing the same string and the same JSON body.

backend/open_webui/constants.py

@@ -99,7 +99,7 @@ class ERROR_MESSAGES(str, Enum):
 
     INVALID_URL = 'The URL you provided is invalid. Please double-check and try again.'
 
-    WEB_SEARCH_ERROR = lambda err='': err if err else 'Something went wrong while searching the web.'
+    WEB_SEARCH_ERROR = 'Something went wrong while searching the web.'
 
     OLLAMA_API_DISABLED = 'The Ollama API is disabled. Please enable it to use this feature.'