Continue sampling after assistant chunks

[surajs-oai]

Summary

• continue sampling when a responses stream ends on reasoning or assistant commentary
• keep the continuation decision scoped to the last completed output item

Test plan

• just fmt
• cargo test -p codex-core (one unrelated zsh snapshot timeout)
• cargo test -p codex-core --lib -- --skip shell_snapshot::tests::macos_zsh_snapshot_includes_sections

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