[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/goals.rs

@@ -1331,12 +1331,9 @@ impl Session {
                 .await;
             return;
         }
-        {
-            let mut turn_state = turn_state.lock().await;
-            for item in candidate.items {
-                turn_state.push_pending_input(item);
-            }
-        }
+        self.input_queue
+            .extend_pending_input_for_turn_state(turn_state.as_ref(), candidate.items)
+            .await;
 
         let turn_context = self
             .new_default_turn_with_sub_id(uuid::Uuid::new_v4().to_string())
@@ -1374,11 +1371,15 @@ impl Session {
             tracing::debug!("skipping active goal continuation because a turn is already active");
             return None;
         }
-        if self.has_queued_response_items_for_next_turn().await {
+        if self
+            .input_queue
+            .has_queued_response_items_for_next_turn()
+            .await
+        {
             tracing::debug!("skipping active goal continuation because queued input exists");
             return None;
         }
-        if self.has_trigger_turn_mailbox_items().await {
+        if self.input_queue.has_trigger_turn_mailbox_items().await {
             tracing::debug!(
                 "skipping active goal continuation because trigger-turn mailbox input is pending"
             );
@@ -1415,8 +1416,11 @@ impl Session {
             return None;
         }
         if self.active_turn.lock().await.is_some()
-            || self.has_queued_response_items_for_next_turn().await
-            || self.has_trigger_turn_mailbox_items().await
+            || self
+                .input_queue
+                .has_queued_response_items_for_next_turn()
+                .await
+            || self.input_queue.has_trigger_turn_mailbox_items().await
         {
             tracing::debug!("skipping active goal continuation because pending work appeared");
             return None;