fix: keep batch-path function_call_output.output as string per OpenAI spec

The streaming path emits output as content-part arrays for Open WebUI
compatibility, but the batch (non-streaming) Responses API path must
return output as a plain string per the OpenAI Responses API spec.
Reverts the _extract_output_items change from the cherry-picked commits
while preserving the streaming path's array format.

gateway/platforms/api_server.py

@@ -1949,15 +1949,10 @@ class APIServerAdapter(BasePlatformAdapter):
                         "call_id": tc.get("id", ""),
                     })
             elif role == "tool":
-                output_content = msg.get("content", "")
-                if isinstance(output_content, list):
-                    output = output_content
-                else:
-                    output = [{"type": "input_text", "text": str(output_content)}]
                 items.append({
                     "type": "function_call_output",
                     "call_id": msg.get("tool_call_id", ""),
-                    "output": output,
+                    "output": msg.get("content", ""),
                 })
 
         # Final assistant message

tests/gateway/test_api_server.py

@@ -1547,7 +1547,7 @@ class TestToolCallsInOutput:
             assert output[0]["call_id"] == "call_abc123"
             assert output[1]["type"] == "function_call_output"
             assert output[1]["call_id"] == "call_abc123"
-            assert output[1]["output"] == [{"type": "input_text", "text": "42"}]
+            assert output[1]["output"] == "42"
             assert output[2]["type"] == "message"
             assert output[2]["content"][0]["text"] == "The result is 42."