core: box multi-agent handler futures (#22266)

## Why

This is the base PR in the split stack for the permissions migration. It
isolates stack-safety work that had been mixed into the larger
permissions PR, so reviewers can evaluate the async-future changes
separately from the permissions model changes in #22267.

The main risk this addresses is large or recursive multi-agent futures
overflowing smaller runner stacks. A follow-up review also called out
that `shutdown_live_agent` must remain quiescent: callers should not
remove a live agent from tracking or release its spawn slot until the
worker loop has actually terminated.

## What Changed

- Boxes the large async futures in the multi-agent spawn, resume, and
close tool handlers.
- Boxes the `AgentControl` spawn and recursive close/shutdown paths that
can otherwise build very deep futures.
- Keeps `shutdown_live_agent` waiting for thread termination before
removing/releasing the live agent, preserving the previous shutdown
ordering while still boxing the recursive close path.

## Verification Strategy

The focused local coverage was `cargo test -p codex-core multi_agents`,
which exercises the multi-agent spawn/resume/close handlers, cascade
close/resume behavior, and the shutdown path touched by this PR.












---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/22266).
* #22330
* #22329
* #22328
* #22327
* __->__ #22266

codex-rs/core/src/agent/control.rs

@@ -233,32 +233,31 @@ impl AgentControl {
         // The same `AgentControl` is sent to spawn the thread.
         let new_thread = match (session_source, options.fork_mode.as_ref()) {
             (Some(session_source), Some(_)) => {
-                self.spawn_forked_thread(
+                Box::pin(self.spawn_forked_thread(
                     &state,
                     config,
                     session_source,
                     &options,
                     inherited_shell_snapshot,
                     inherited_exec_policy,
-                )
+                ))
                 .await?
             }
             (Some(session_source), None) => {
-                state
-                    .spawn_new_thread_with_source(
-                        config.clone(),
-                        self.clone(),
-                        session_source,
-                        /*thread_source*/ Some(ThreadSource::Subagent),
-                        /*persist_extended_history*/ false,
-                        /*metrics_service_name*/ None,
-                        inherited_shell_snapshot,
-                        inherited_exec_policy,
-                        options.environments.clone(),
-                    )
-                    .await?
+                Box::pin(state.spawn_new_thread_with_source(
+                    config.clone(),
+                    self.clone(),
+                    session_source,
+                    /*thread_source*/ Some(ThreadSource::Subagent),
+                    /*persist_extended_history*/ false,
+                    /*metrics_service_name*/ None,
+                    inherited_shell_snapshot,
+                    inherited_exec_policy,
+                    options.environments.clone(),
+                ))
+                .await?
             }
-            (None, _) => state.spawn_new_thread(config.clone(), self.clone()).await?,
+            (None, _) => Box::pin(state.spawn_new_thread(config.clone(), self.clone())).await?,
         };
         agent_metadata.agent_id = Some(new_thread.thread_id);
         reservation.commit(agent_metadata.clone());