[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/state/turn.rs

@@ -11,7 +11,6 @@ use tokio_util::task::AbortOnDropHandle;
 
 use codex_extension_api::ExtensionData;
 use codex_protocol::dynamic_tools::DynamicToolResponse;
-use codex_protocol::models::ResponseInputItem;
 use codex_protocol::request_permissions::RequestPermissionProfile;
 use codex_protocol::request_permissions::RequestPermissionsResponse;
 use codex_protocol::request_user_input::RequestUserInputResponse;
@@ -20,6 +19,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use rmcp::model::RequestId;
 use tokio::sync::oneshot;
 
+use crate::session::TurnInputQueue;
 use crate::session::turn_context::TurnContext;
 use crate::tasks::AnySessionTask;
 use codex_protocol::models::AdditionalPermissionProfile;
@@ -115,7 +115,7 @@ pub(crate) struct TurnState {
     pending_user_input: HashMap<String, oneshot::Sender<RequestUserInputResponse>>,
     pending_elicitations: HashMap<(String, RequestId), oneshot::Sender<ElicitationResponse>>,
     pending_dynamic_tools: HashMap<String, oneshot::Sender<DynamicToolResponse>>,
-    pending_input: Vec<ResponseInputItem>,
+    pub(crate) pending_input: TurnInputQueue,
     mailbox_delivery_phase: MailboxDeliveryPhase,
     granted_permissions: Option<AdditionalPermissionProfile>,
     strict_auto_review_enabled: bool,
@@ -146,13 +146,12 @@ impl TurnState {
         self.pending_approvals.remove(key)
     }
 
-    pub(crate) fn clear_pending(&mut self) {
+    pub(crate) fn clear_pending_waiters(&mut self) {
         self.pending_approvals.clear();
         self.pending_request_permissions.clear();
         self.pending_user_input.clear();
         self.pending_elicitations.clear();
         self.pending_dynamic_tools.clear();
-        self.pending_input.clear();
     }
 
     pub(crate) fn insert_pending_request_permissions(
@@ -220,33 +219,6 @@ impl TurnState {
         self.pending_dynamic_tools.remove(key)
     }
 
-    pub(crate) fn push_pending_input(&mut self, input: ResponseInputItem) {
-        self.pending_input.push(input);
-    }
-
-    pub(crate) fn prepend_pending_input(&mut self, mut input: Vec<ResponseInputItem>) {
-        if input.is_empty() {
-            return;
-        }
-
-        input.append(&mut self.pending_input);
-        self.pending_input = input;
-    }
-
-    pub(crate) fn take_pending_input(&mut self) -> Vec<ResponseInputItem> {
-        if self.pending_input.is_empty() {
-            Vec::with_capacity(0)
-        } else {
-            let mut ret = Vec::new();
-            std::mem::swap(&mut ret, &mut self.pending_input);
-            ret
-        }
-    }
-
-    pub(crate) fn has_pending_input(&self) -> bool {
-        !self.pending_input.is_empty()
-    }
-
     pub(crate) fn accept_mailbox_delivery_for_current_turn(&mut self) {
         self.set_mailbox_delivery_phase(MailboxDeliveryPhase::CurrentTurn);
     }
@@ -276,11 +248,3 @@ impl TurnState {
         self.strict_auto_review_enabled
     }
 }
-
-impl ActiveTurn {
-    /// Clear any pending approvals and input buffered for the current turn.
-    pub(crate) async fn clear_pending(&self) {
-        let mut ts = self.turn_state.lock().await;
-        ts.clear_pending();
-    }
-}