Fix agent jobs finalization race and reduce status polling churn

[daveaitel-openai]

Summary

• make report_agent_job_result atomically transition an item from running to completed while storing result_json
• remove brittle finalization grace-sleep logic and make finished-item cleanup idempotent
• replace blind fixed-interval waiting with status-subscription-based waiting for active worker threads
• add state runtime tests for atomic completion and late-report rejection

Why

This addresses the race and polling concerns in #13948 by removing timing-based correctness assumptions and reducing unnecessary status polling churn.

Validation

• cd codex-rs && just fmt
• cd codex-rs && cargo test -p codex-state
• cd codex-rs && cargo test -p codex-core --test all suite::agent_jobs
• cd codex-rs && cargo test
  • fails in an unrelated app-server tracing test: message_processor::tracing_tests::thread_start_jsonrpc_span_exports_server_span_and_parents_children timed out waiting for response

Notes

• This PR supersedes Fix agent jobs finalization race and reduce status polling churn #14129 with the same agent-jobs fix on a clean branch from main.
• The earlier PR branch was stacked on unrelated history, which made the review diff include unrelated commits.

Fixes #13948