feat(proxy_server.py): allow admin to return rejected response as string to user

Closes https://github.com/BerriAI/litellm/issues/3671

litellm/utils.py

@@ -12187,3 +12187,29 @@ def _add_key_name_and_team_to_alert(request_info: str, metadata: dict) -> str:
         return request_info
     except:
         return request_info
+
+
+class ModelResponseIterator:
+    def __init__(self, model_response):
+        self.model_response = model_response
+        self.is_done = False
+
+    # Sync iterator
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.is_done:
+            raise StopIteration
+        self.is_done = True
+        return self.model_response
+
+    # Async iterator
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self):
+        if self.is_done:
+            raise StopAsyncIteration
+        self.is_done = True
+        return self.model_response