Reload Multi-Agent V2 children through their parent (#40477)

## Why

Multi-Agent V2 children are owned by their parent at runtime. Resuming an
unloaded child directly could rebuild it from caller-provided settings instead
of the parent's current authority.

## What changed

- Route child reloads through their loaded immediate parent and reject direct
  cold resume when that parent is unavailable.
- Preserve the child's recorded model, provider, reasoning effort, and role
  while inheriting the parent's execution policy and MCP extensions.
- Validate cached environments against the parent, intersect local permission
  profiles, and reject stale remote or workspace-root configurations.
- Keep paginated cold-resume token usage attribution when attaching to the
  reloaded child.

## Testing

Add coverage for owner validation, nested-parent reloads, permission and
environment changes, role instruction restoration, paginated usage replay,
and app-server child resume behavior.

GitOrigin-RevId: 84014762bcdb57f94fc23974c9c1af55dc1babf1

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

@@ -52,7 +52,7 @@ impl Handler {
             session
                 .services
                 .agent_control
-                .ensure_v2_agent_loaded(resume_config, receiver_thread_id)
+                .ensure_v2_agent_loaded(resume_config, receiver_thread_id, /*parent*/ None)
                 .await
                 .map_err(|err| collab_agent_error(receiver_thread_id, err))?;
         }

codex-rs/core/src/tools/handlers/multi_agents_v2/message_tool.rs

@@ -86,7 +86,7 @@ pub(crate) async fn handle_message_string_tool(
     session
         .services
         .agent_control
-        .ensure_v2_agent_loaded(resume_config, receiver_thread_id)
+        .ensure_v2_agent_loaded(resume_config, receiver_thread_id, /*parent*/ None)
         .await
         .map_err(|err| collab_agent_error(receiver_thread_id, err))?;
     let author = turn

codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs

@@ -150,11 +150,20 @@ async fn handle_spawn_agent(
     .await?;
     apply_spawn_agent_runtime_overrides(&mut config, turn.as_ref())?;
 
+    // Remember an applied configured default so cold reload reapplies its restrictions.
+    let persisted_role_name = role_name.or_else(|| {
+        (!is_full_history_fork
+            && config
+                .agent_roles
+                .get(DEFAULT_ROLE_NAME)
+                .is_some_and(|role| role.config_file.is_some()))
+        .then_some(DEFAULT_ROLE_NAME)
+    });
     let spawn_source = thread_spawn_source(
         session.thread_id,
         &turn.session_source,
         child_depth,
-        role_name,
+        persisted_role_name,
         Some(args.task_name.clone()),
     )?;
     let new_agent_path = spawn_source.get_agent_path().ok_or_else(|| {