[Manohar Paturi (ManoharPaturi)]

Motivation & Context

The conversation items endpoints disagreed about identity. add_items mints item_<uuid> ids and get_item resolved only those, while list_items derives positional item_<index> ids from the stored messages. The only ids a client can discover through listing, including agent run messages that never pass through the add path, answered 404 on the item retrieval endpoint that documents itself as the OpenAI standard.

Description & Review Guide

• What are the major changes? The per message conversion that list_items used inline is extracted into one helper, and get_item resolves positional ids by rebuilding the addressed message's items with that same helper, covering the item_<index>, item_<index>_call_<call id>, and item_<index>_result_<call id> shapes the list emits.
• What is the impact of these changes? Listing and retrieval now agree on ids and content by construction because both render through the same conversion. The add path's uuid ids keep resolving exactly as before, and an unknown or out of range index still returns None.
• What do you want reviewers to focus on? The helper extraction being behavior preserving for the list output, and the positional resolution reading the stored messages directly so it also covers messages written by agent runs.

Related Issue

Fixes #9862

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Contribution Checklist

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[Manohar Paturi (ManoharPaturi)]

Understood, my mistake for opening this before the issue went through review. #9862 now follows the bug report template with the reproduction, and this PR stays closed until the team agrees.