core: box multi-agent wrapper futures

[bolinfest]

Why

While debugging the Windows stack overflows we saw in #13429 and then again in #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)

[eternal-openai]

<3