core: box multi-agent wrapper futures (#19059)

## Why

While debugging the Windows stack overflows we saw in
[#13429](https://github.com/openai/codex/pull/13429) and then again in
[#18893](https://github.com/openai/codex/pull/18893), I hit another
overflow in
`tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed`.

That test drives the legacy multi-agent spawn / close / resume path. The
behavior was fine, but several thin async wrappers were still inlining
much larger `AgentControl` futures into their callers, which was enough
to overflow the default Windows stack.

## What

- Box the thin `AgentControl` wrappers around `spawn_agent_internal`,
`resume_single_agent_from_rollout`, and `shutdown_agent_tree`.
- Box the corresponding legacy `multi_agents` handler calls in `spawn`,
`resume_agent`, and `close_agent`.
- Keep behavior unchanged while reducing future size on this call path
so the Windows test no longer overflows its stack.

## Testing

- `cargo test -p codex-core --lib
tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed
-- --exact --nocapture`
- `cargo test -p codex-core` (this still hit unrelated local
integration-test failures because `codex.exe` / `test_stdio_server.exe`
were not present in this shell; the relevant unit tests passed)

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

@@ -163,15 +163,14 @@ impl AgentControl {
         initial_operation: Op,
         session_source: Option<SessionSource>,
     ) -> CodexResult<ThreadId> {
-        Ok(self
-            .spawn_agent_internal(
-                config,
-                initial_operation,
-                session_source,
-                SpawnAgentOptions::default(),
-            )
-            .await?
-            .thread_id)
+        let spawned_agent = Box::pin(self.spawn_agent_internal(
+            config,
+            initial_operation,
+            session_source,
+            SpawnAgentOptions::default(),
+        ))
+        .await?;
+        Ok(spawned_agent.thread_id)
     }
 
     /// Spawn an agent thread with some metadata.
@@ -182,7 +181,7 @@ impl AgentControl {
         session_source: Option<SessionSource>,
         options: SpawnAgentOptions, // TODO(jif) drop with new fork.
     ) -> CodexResult<LiveAgent> {
-        self.spawn_agent_internal(config, initial_operation, session_source, options)
+        Box::pin(self.spawn_agent_internal(config, initial_operation, session_source, options))
             .await
     }
 
@@ -418,9 +417,12 @@ impl AgentControl {
         session_source: SessionSource,
     ) -> CodexResult<ThreadId> {
         let root_depth = thread_spawn_depth(&session_source).unwrap_or(0);
-        let resumed_thread_id = self
-            .resume_single_agent_from_rollout(config.clone(), thread_id, session_source)
-            .await?;
+        let resumed_thread_id = Box::pin(self.resume_single_agent_from_rollout(
+            config.clone(),
+            thread_id,
+            session_source,
+        ))
+        .await?;
         let state = self.upgrade()?;
         let Ok(resumed_thread) = state.get_thread(resumed_thread_id).await else {
             return Ok(resumed_thread_id);
@@ -698,7 +700,7 @@ impl AgentControl {
         {
             warn!("failed to persist thread-spawn edge status for {agent_id}: {err}");
         }
-        self.shutdown_agent_tree(agent_id).await
+        Box::pin(self.shutdown_agent_tree(agent_id)).await
     }
 
     /// Shut down `agent_id` and any live descendants reachable from the in-memory spawn tree.

codex-rs/core/src/tools/handlers/multi_agents/close_agent.rs

@@ -66,10 +66,7 @@ impl ToolHandler for Handler {
                 return Err(collab_agent_error(agent_id, err));
             }
         };
-        let result = session
-            .services
-            .agent_control
-            .close_agent(agent_id)
+        let result = Box::pin(session.services.agent_control.close_agent(agent_id))
             .await
             .map_err(|err| collab_agent_error(agent_id, err))
             .map(|_| ());

codex-rs/core/src/tools/handlers/multi_agents/resume_agent.rs

@@ -61,7 +61,14 @@ impl ToolHandler for Handler {
             .get_status(receiver_thread_id)
             .await;
         let (receiver_agent, error) = if matches!(status, AgentStatus::NotFound) {
-            match try_resume_closed_agent(&session, &turn, receiver_thread_id, child_depth).await {
+            match Box::pin(try_resume_closed_agent(
+                &session,
+                &turn,
+                receiver_thread_id,
+                child_depth,
+            ))
+            .await
+            {
                 Ok(()) => {
                     status = session
                         .services
@@ -149,21 +156,18 @@ async fn try_resume_closed_agent(
     child_depth: i32,
 ) -> Result<(), FunctionCallError> {
     let config = build_agent_resume_config(turn.as_ref(), child_depth)?;
-    session
-        .services
-        .agent_control
-        .resume_agent_from_rollout(
-            config,
-            receiver_thread_id,
-            thread_spawn_source(
-                session.conversation_id,
-                &turn.session_source,
-                child_depth,
-                /*agent_role*/ None,
-                /*task_name*/ None,
-            )?,
-        )
-        .await
-        .map(|_| ())
-        .map_err(|err| collab_agent_error(receiver_thread_id, err))
+    Box::pin(session.services.agent_control.resume_agent_from_rollout(
+        config,
+        receiver_thread_id,
+        thread_spawn_source(
+            session.conversation_id,
+            &turn.session_source,
+            child_depth,
+            /*agent_role*/ None,
+            /*task_name*/ None,
+        )?,
+    ))
+    .await
+    .map(|_| ())
+    .map_err(|err| collab_agent_error(receiver_thread_id, err))
 }

codex-rs/core/src/tools/handlers/multi_agents/spawn.rs

@@ -82,26 +82,23 @@ impl ToolHandler for Handler {
         apply_spawn_agent_runtime_overrides(&mut config, turn.as_ref())?;
         apply_spawn_agent_overrides(&mut config, child_depth);
 
-        let result = session
-            .services
-            .agent_control
-            .spawn_agent_with_metadata(
-                config,
-                input_items,
-                Some(thread_spawn_source(
-                    session.conversation_id,
-                    &turn.session_source,
-                    child_depth,
-                    role_name,
-                    /*task_name*/ None,
-                )?),
-                SpawnAgentOptions {
-                    fork_parent_spawn_call_id: args.fork_context.then(|| call_id.clone()),
-                    fork_mode: args.fork_context.then_some(SpawnAgentForkMode::FullHistory),
-                },
-            )
-            .await
-            .map_err(collab_spawn_error);
+        let result = Box::pin(session.services.agent_control.spawn_agent_with_metadata(
+            config,
+            input_items,
+            Some(thread_spawn_source(
+                session.conversation_id,
+                &turn.session_source,
+                child_depth,
+                role_name,
+                /*task_name*/ None,
+            )?),
+            SpawnAgentOptions {
+                fork_parent_spawn_call_id: args.fork_context.then(|| call_id.clone()),
+                fork_mode: args.fork_context.then_some(SpawnAgentForkMode::FullHistory),
+            },
+        ))
+        .await
+        .map_err(collab_spawn_error);
         let (new_thread_id, new_agent_metadata, status) = match &result {
             Ok(spawned_agent) => (
                 Some(spawned_agent.thread_id),