[codex] Move pending input into input queue (#22728)

## 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.

codex-rs/core/src/stream_events_utils.rs

@@ -158,7 +158,8 @@ pub(crate) async fn record_completed_response_item_with_finalized_facts(
         |facts| facts.defers_mailbox_delivery_to_next_turn,
     );
     if defers_mailbox_delivery {
-        sess.defer_mailbox_delivery_to_next_turn(&turn_context.sub_id)
+        sess.input_queue
+            .defer_mailbox_delivery_to_next_turn(&sess.active_turn, &turn_context.sub_id)
             .await;
     }
     mark_thread_memory_mode_polluted_if_external_context(sess, turn_context, item).await;
@@ -351,7 +352,11 @@ pub(crate) async fn handle_output_item_done(
         // The model emitted a tool call; log it, persist the item immediately, and queue the tool execution.
         Ok(Some(call)) => {
             ctx.sess
-                .accept_mailbox_delivery_for_current_turn(&ctx.turn_context.sub_id)
+                .input_queue
+                .accept_mailbox_delivery_for_current_turn(
+                    &ctx.sess.active_turn,
+                    &ctx.turn_context.sub_id,
+                )
                 .await;
 
             let payload_preview = call.payload.log_payload().into_owned();