[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/session/turn.rs

@@ -148,7 +148,7 @@ pub(crate) async fn run_turn(
     prewarmed_client_session: Option<ModelClientSession>,
     cancellation_token: CancellationToken,
 ) -> Option<String> {
-    if input.is_empty() && !sess.has_pending_input().await {
+    if input.is_empty() && !sess.input_queue.has_pending_input(&sess.active_turn).await {
         return None;
     }
 
@@ -413,7 +413,7 @@ pub(crate) async fn run_turn(
         // submitted through the UI while the model was running. Though the UI
         // may support this, the model might not.
         let pending_input = if can_drain_pending_input {
-            sess.get_pending_input().await
+            sess.input_queue.get_pending_input(&sess.active_turn).await
         } else {
             Vec::new()
         };
@@ -434,7 +434,10 @@ pub(crate) async fn run_turn(
                     } => {
                         let remaining_pending_input = pending_input_iter.collect::<Vec<_>>();
                         if !remaining_pending_input.is_empty() {
-                            let _ = sess.prepend_pending_input(remaining_pending_input).await;
+                            let _ = sess
+                                .input_queue
+                                .prepend_pending_input(&sess.active_turn, remaining_pending_input)
+                                .await;
                             requeued_pending_input = true;
                         }
                         blocked_pending_input_contexts = additional_contexts;
@@ -494,7 +497,7 @@ pub(crate) async fn run_turn(
                     last_agent_message: sampling_request_last_agent_message,
                 } = sampling_request_output;
                 can_drain_pending_input = true;
-                let has_pending_input = sess.has_pending_input().await;
+                let has_pending_input = sess.input_queue.has_pending_input(&sess.active_turn).await;
                 let needs_follow_up = model_needs_follow_up || has_pending_input;
                 let total_usage_tokens = sess.get_total_token_usage().await;
                 let token_limit_reached = total_usage_tokens >= auto_compact_limit;
@@ -2065,7 +2068,7 @@ async fn try_run_sampling_request(
                 }
                 needs_follow_up |= output_result.needs_follow_up;
                 // todo: remove before stabilizing multi-agent v2
-                if preempt_for_mailbox_mail && sess.mailbox_rx.lock().await.has_pending() {
+                if preempt_for_mailbox_mail && sess.input_queue.has_pending_mailbox_items().await {
                     break Ok(SamplingRequestResult {
                         needs_follow_up: true,
                         last_agent_message,