fix: Deprecate items when create conversation

[gyliu513]

What does this PR do?

Fixed #4718

[mattf]

the items should not be returned when retrieving the conversation

[franciscojavierarceo]

@mattf +1 this was a bug in my original PR #3429

[gyliu513]

thanks @franciscojavierarceo and @mattf , seeme we can just remove items from coversation, but end user can query item via conversations/:id/items, this also align with OpenAI spec, so I think I need to remove items when retrive conversation, comments?

GET /v1/conversations/{conversation_id} ⁠

{
  "id": "conv_123",
  "object": "conversation",
  "created_at": 1741900000,
  "metadata": {"topic": "demo"}
}

Create conversation

{
  "id": "conv_123",
  "object": "conversation",
  "created_at": 1741900000,
  "metadata": {"topic": "demo"}
}