Do not run the model twice when Conversation API completion retries

[dcramer]

Problem

The Conversation API worker acknowledges input during a Run. The model can finish before Junior finishes saving the reply, Conversation state, checkpoint, and Turn event. If one of those later writes fails, a queue retry can run the model again and repeat work.

This behavior existed before #1682. That refactor must not hide it, but it must not add a new recovery design either.

Required behavior

• After the model finishes, a retry must finish saving the result. It must not start another model Run.
• Preserve the result: success, intentional no reply, or the original model failure code and event ID.
• Store one visible reply. If recovery starts before the failure reply is stored, store the standard event-ID failure reply once.
• Run plugin work after a recovered success or intentional no reply. Do not run it after a failure.
• Leave the Conversation ready for a later Turn.

Design limits

• Core stores only the result data needed to finish safely. This data must not contain sensitive model output.
• The Conversation API code owns its reply and Conversation state recovery.
• Do not store raw model output or exception text in retry state.
• Do not use assistant message metadata as another Turn result store.
• Do not add a provider framework, a service options bag, or a compatibility name.
• Keep this separate from the executeTurn adoption in ref(chat): use native Turn execution for Conversation API #1682 and the later work in Simplify Junior around one core Conversation and optional Location #1563.

Tests

Extend the main Conversation API integration scenario. Use the real queue, worker, and agent. Fake only model output.

Assert one model Run, one visible result, the correct final Turn event, plugin work after recovered success, and a normal following Turn. Add a component test only if a separate stored-result rule cannot be tested through that integration path.

Review findings

• ref(chat): use native Turn execution for Conversation API #1682 (comment)
• ref(chat): use native Turn execution for Conversation API #1682 (comment)