code-mode: preserve initial yield at completion

[cconger]

Summary

• Retain the first pre-observation yield_control() boundary when a cell completes before observation.
• Deliver the preserved yield before the buffered completion.
• Keep later unattached yields as no-ops.

Why

Create followed by the initial wait must preserve the former execute response boundary even when the script runs to completion first.

Impact

The first wait observes the same initial yield boundary as before create and observe were decoupled.

Validation

• Focused initial-yield signature regression passed.
• Stack-tip validation: just test -p codex-code-mode -p codex-code-mode-protocol (70 passed).
• Parent branch: cconger/code-mode-runtime-compact-03e2-observation-delivery.

[jif-oai]

This slice never supplies a real initial yield: both production completion call sites still pass None, and only the direct state tests construct Some(...)
Is this expected? This does not seems aligned with the PR title

[cconger]

This is unfortunately preparatory for #29290 they could likely have been merged.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the first dropped yield at completion

When the initial YieldAfter response is dropped before the actor can deliver a yield_control() boundary and the cell then runs to completion before another observation, this path still commits the completion with pending_initial_yield_items set to None. The failed yield delivery is only folded back into content_items, so the next YieldAfter observe receives one merged Completed event instead of the preserved Yielded boundary followed by the buffered completion; the new CellState buffering is only exercised by tests that call commit_completion(..., Some(...)) directly, not by the actor. Capture the first undelivered initial yield, including an empty yield, and pass it into the completion commit here.

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[cconger]

We address this later, good catch though.