Handle exec shutdown on Interrupt (fixes immortal codex exec with websockets)

[rasmusrygaard]

Motivation

• Ensure codex exec exits when a running turn is interrupted (e.g., Ctrl-C) so the CLI is not "immortal" when websockets/streaming are used.

Description

• Return CodexStatus::InitiateShutdown when handling EventMsg::TurnAborted in exec/src/event_processor_with_human_output.rs so human-output exec mode shuts down after an interrupt.
• Treat protocol::EventMsg::TurnAborted as CodexStatus::InitiateShutdown in exec/src/event_processor_with_jsonl_output.rs so JSONL output mode behaves the same.
• Applied formatting with just fmt.

Testing

• Ran just fmt successfully.
• Ran cargo test -p codex-exec; many unit tests ran and the test command completed, but the full test run in this environment produced 35 passed, 11 failed where the failures are due to Landlock sandbox panics and 403 responses in the test harness (environmental/integration issues) and are not caused by the interrupt/shutdown changes.

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Codex Task