[codex] Move pending input into input queue

[pakrym-oai]

Why

Pending model input was split across Session, TurnState, and the agent mailbox. That made it easy for new paths to manage queued user input or mailbox delivery outside the intended ownership boundary.

This PR consolidates the model-facing input lifecycle behind the session input queue so turn-local pending input, next-turn queued items, and mailbox delivery coordination are owned in one place.

What Changed

• Added session/input_queue.rs to own pending input queues and mailbox delivery coordination.
• Removed the standalone agent/mailbox.rs channel wrapper and store mailbox items directly in the input queue.
• Moved pending-input mutations off TurnState; TurnState now exposes the queue-owned storage directly for now.
• Routed abort cleanup, mailbox delivery phase changes, next-turn queued items, and active-turn pending input through InputQueue.
• Boxed stack-heavy agent resume/fork startup futures that the refactor pushed over the default test stack.
• Updated session, task, goal, stream-event, and multi-agent call sites and tests to use the new queue ownership.

Verification

• cargo test -p codex-core --lib agent::control::tests
• cargo test -p codex-core --lib agent::control::tests::resume_closed_child_reopens_open_descendants -- --exact
• cargo test -p codex-core --lib agent::control::tests::spawn_agent_fork_last_n_turns_keeps_only_recent_turns -- --exact
• cargo test -p codex-core --lib agent::control::tests::resume_thread_subagent_restores_stored_nickname_and_role -- --exact
• cargo test -p codex-core was also run; it completed with 1814 passed, 4 ignored, and one timeout in agent::control::tests::resume_thread_subagent_restores_stored_nickname_and_role, which passed when rerun in isolation.