[codex] Use TurnInput for session task input

[pakrym-oai]

Why

The idea here is to erase the difference between initial and followup inputs to a turn. Followup inputs are already represented as TurnInput.

Eventual goal is not to have explicit on task input at all and pull everything from input Q.

What Changed

• Changes SessionTask::run and the erased AnySessionTask::run path to accept Vec<TurnInput>.
• Wraps user-submitted spawn input as TurnInput::UserInput at the session task start boundary.
• Updates run_turn to record initial TurnInput using the same hook and recording path used for pending input.
• Keeps review-specific conversion local to ReviewTask, where the sub-Codex one-shot API still expects Vec<UserInput>.
• Moves the synthetic compact prompt into CompactTask and starts compact tasks with empty task input.

Validation

• cargo check -p codex-core
• just test -p codex-core -E 'test(task_finish_emits_turn_item_lifecycle_for_leftover_pending_user_input) | test(queued_response_items_for_next_turn_move_into_next_active_turn) | test(steered_input_reopens_mailbox_delivery_for_current_turn)'