fix(core): preserve terminal subagent errors

[shiny-code-bot]

Summary

• retain status-affecting subagent errors through trailing TurnComplete and TurnAborted events
• install the retained Errored status before terminal event delivery, including legacy errors without typed metadata
• deduplicate MultiAgentV2 parent notifications across completion/abort races
• add manager-level and mocked stream regressions for child status and parent delivery

Context

Current main already distinguishes retryable StreamError events from fatal errors. The remaining failure mode was manager-level reconciliation: a fatal ErrorEvent could be overwritten by the generic trailing TurnComplete(None), making a failed child look like an empty success.

This ports the relevant terminal-error precedence behavior from upstream while adapting it to this fork's current event-delivery and direct-parent notification paths.

Validation

• just fmt
• cargo test -p codex-core multi_agent_v2_terminal_error_queues_message_for_direct_parent --lib -- --nocapture
• cargo test -p codex-core --test all multi_agent_v2_terminal_stream_error_preserves_errored_status -- --nocapture
• cargo test -p codex-core agent::control::tests --lib -- --test-threads=1 --nocapture
• cargo test -p codex-core --test all suite::subagent_notifications -- --test-threads=1 --nocapture
• ./build-fast.sh
• git diff --check

JetBrains closeout inspection reported no problems, but its verdict was inconclusive because the configured IntelliJ instance did not provide semantic Rust coverage.

Refs #434