bug: /conversations/{id}/items response format incompatible with OpenAI spec

[louis-jan]

Title: /conversations/{id}/items response format incompatible with OpenAI spec

---

The response structure from /conversations/{id}/items does not match the expected OpenAI-compatible format, causing parsing issues for clients expecting standard responses.

Issues

1. Nested text object in assistant responses

Currently returns:

{
  "text": {
    "text": "response content"
  }
}

Expected:

{
  "text": "response content"
}

2. Incorrect content type values

Current	Expected
input_text	text
reasoning_content->reasoning_text	text

Impact

Clients built against the OpenAI spec need additional transformation logic to handle these differences, breaking drop-in compatibility.