#10838 Fix premature Codex completion notifications

Summary

• Add behavior regressions for nested Codex reviewer Stops, native child lifecycle, exact final completion, and normal top-level Stops.
• Supersedes the approach in #9512 without modifying that open PR.

Fixes #7520

Regression provenance

The first commit (be5ee2a567) intentionally adds executable behavior tests before the implementation. CI should fail until the ownership/settlement implementation lands.

Testing

• Pending CI red baseline for CodexTurnCompletionOwnershipTests.
• Implementation and final CI proof will follow in a separate commit.

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Summary by CodeRabbit

• New Features

  • Improved Codex turn tracking for nested agents and child processes.
  • Completion notifications and runtime status now reflect child lifecycle events more accurately.
  • Prevented duplicate or premature completion notifications while child work remains active.
  • Added reliable handling for Codex subagent start and stop events.
  • Improved persistence and recovery of Codex turn state.

• Tests

  • Added coverage for nested processes, child-agent activity, exactly-once completion, and standard top-level stops.