Add acknowledged user message submission to core

[copyberry]

Add acknowledged user message submission to core

What changed

• Add CodexThread::submit_user_input_and_wait_for_admission and export the
  UserMessageAdmission result through codex-core-api.
• Resolve submissions only after they start a new turn or steer the active
  turn, returning the accepting turn ID in either case.
• Return errors for invalid operations, rejected thread settings, and session
  termination instead of leaving admission waiters unresolved.

Testing

Add integration coverage for concurrent start-and-steer submissions, settings
rejection and recovery, non-user operations, and submission after shutdown.

[github-actions]

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