[codex] Make active turn task singular

[pakrym-oai]

Why

ActiveTurn already runs at most one task: starting a task requires that no task is present, and replacement aborts existing work first. Representing that state as an IndexMap leaves a multi-task shape for a single-task invariant and makes each lifecycle lookup operate like a collection lookup.

The slot remains optional because goal continuation uses an empty active turn as a reservation while deciding whether to start continuation work.

What changed

• Replace ActiveTurn.tasks with task: Option<RunningTask>.
• Update task abort/completion, session lookup and steering, input-queue matching, goal reservation, and network-approval lookup to operate on the singular slot.
• Mutate the singular task slot directly instead of retaining collection-era add/remove/take helpers.
• Record token usage on the completing active task span without a regular-task-only opt-in flag.

Validation

• cargo test -p codex-core --lib session::tests::steer_input
• cargo test -p codex-core --lib session::tests::abort_empty_active_turn_preserves_pending_input
• cargo test -p codex-core --lib session::tests::queued_response_items_for_next_turn_move_into_next_active_turn
• cargo test -p codex-core --lib session::tests::active_goal_continuation_runs_again_after_no_tool_turn
• cargo test -p codex-core --lib session::tests::abort_regular_task_emits_turn_aborted_only
• cargo test -p codex-core --lib session::input_queue::tests

[pakrym-oai]

@codex review this

[chatgpt-codex-connector]

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[jif-oai]

Can we conditionally install only if the reserved turn state is still current, and otherwise bail/cleanup?

start_task has awaits between the empty active-turn reservation and installing the task, so if that reservation gets stolen by a user turn, this assignment can overwrite the new active task in release builds.
I don't think this can happen right now but it would be better to be loud/clean if this primitive changes in the future (because we know how stable primitives are...)