fix(guardrails): tolerate a model-less request when translating Anthropic response context

The proxy-endpoints shard failed with KeyError: 'model' because the new Anthropic post-call context translation reached translate_anthropic_to_openai with request data that only carried messages and guardrail metadata.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py

@@ -1180,7 +1180,7 @@ class LiteLLMAnthropicMessagesAdapter:
         self._add_system_message_to_messages(new_messages, anthropic_message_request)
 
         new_kwargs: Final[ChatCompletionRequest] = {
-            "model": anthropic_message_request["model"],
+            "model": anthropic_message_request.get("model", ""),
             "messages": new_messages,
         }
         ## CONVERT METADATA (user_id + litellm metadata)

tests/test_litellm/llms/anthropic/chat/guardrail_translation/test_anthropic_guardrail_handler.py

@@ -2793,3 +2793,19 @@ class TestAnthropicResponseScanCarriesRequestConversation:
         ]
         assert inputs["structured_messages"][-1] == {"role": "assistant", "content": "Paris is the capital"}
         assert inputs["tools"][0]["function"]["name"] == "run_shell"
+
+    @pytest.mark.asyncio
+    async def test_streaming_response_scan_survives_a_request_without_a_model(self):
+        handler = AnthropicMessagesHandler()
+        guardrail = TypedInputsRecordingGuardrail()
+        request = {key: value for key, value in self._request().items() if key != "model"}
+
+        await handler.process_output_streaming_response(
+            responses_so_far=self._sse_chunks(ended=True),
+            guardrail_to_apply=guardrail,
+            litellm_logging_obj=MagicMock(),
+            request_data=request,
+        )
+
+        [(_, inputs)] = guardrail.seen
+        assert [m["role"] for m in inputs["structured_messages"]] == ["system", "user", "assistant", "tool", "assistant"]