fix: preserve explicit timeout=0 in responses API handler

Use 'timeout if timeout is not None else request_timeout' instead of
'timeout or request_timeout' so an explicit timeout=0/0.0 isn't silently
replaced by the default request_timeout.

Co-authored-by: Yassin Kortam <yassin@berri.ai>

litellm/responses/main.py

@@ -1115,7 +1115,7 @@ def responses(
                 stream=stream,
                 extra_headers=extra_headers,
                 extra_body=extra_body,
-                timeout=timeout or request_timeout,
+                timeout=timeout if timeout is not None else request_timeout,
                 **kwargs,
             )