test: fix multi-agent service tier assertion

[bolinfest]

Why

openai/codex#22169 added a regression test that expects an invalid child service_tier to be rejected, but the test used Result::expect_err on SpawnAgentHandler::handle. That requires the Ok type to implement Debug, and this handler returns Box<dyn ToolOutput>, so Bazel failed while compiling codex-core tests before it could run them.

What changed

• Capture the handler result and assert on result.err() instead of calling expect_err.
• Keep the same FunctionCallError::RespondToModel assertion for the rejected service tier.

Verification

• cargo test -p codex-core spawn_agent_role_service_tier_does_not_hide_invalid_spawn_request