fix: assistant input content must be output_text, not input_text

Live /v1/responses rejects input_text for the assistant role
(input[N].content[0] -> "Supported values are: 'output_text' and
'refusal'"). Revert the previous review-driven change: encode assistant
turns as output_text and user/system turns as input_text. The SDK type
union suggested input_* only, but the API enforces output_text per-role —
confirmed against the live endpoint.

application/llm/openai.py

@@ -461,15 +461,12 @@ class OpenAILLM(BaseLLM):
     @staticmethod
     def _responses_content_parts(role, content):
         """Translate a cleaned chat ``content`` value into Responses content
-        parts (``input_text``/``input_image``/``input_file``).
-
-        Assistant text from prior turns is replayed as ``input_text`` too: a
-        Responses easy-input message only accepts the input_* content types
-        regardless of role (``output_text`` is reserved for full
-        output-message items, which carry id/status/annotations).
+        parts. The Responses API enforces the content-part type by message
+        role: assistant turns require ``output_text`` (``input_text`` is
+        rejected with a 400), while user/system turns require ``input_text``.
+        Images/files use ``input_image``/``input_file``.
         """
-        _ = role
-        text_type = "input_text"
+        text_type = "output_text" if role == "assistant" else "input_text"
         parts = []
         if content is None:
             return parts

tests/llm/test_openai_responses.py

@@ -102,7 +102,9 @@ def test_to_responses_input_tool_roundtrip(monkeypatch):
             "arguments": '{"q":"x"}',
         },
         {"type": "function_call_output", "call_id": "call_1", "output": "result"},
-        {"role": "assistant", "content": [{"type": "input_text", "text": "final"}]},
+        # The Responses API requires output_text (not input_text) for the
+        # assistant role; input_text 400s. Locked in here so it can't regress.
+        {"role": "assistant", "content": [{"type": "output_text", "text": "final"}]},
     ]