Fix Codex launch planner shutdown ordering

[danshapiro]

Summary

• Start Codex runtime teardown concurrently with proxy close during launch-planner sidecar shutdown.
• Add a proxy factory seam so launch-planner unit tests do not depend on real WebSocket proxy close timing.
• Add deterministic coverage for slow proxy close during planner shutdown.

Verification

• npm run test:vitest -- test/unit/server/coding-cli/codex-app-server/launch-planner.test.ts test/unit/server/coding-cli/codex-app-server/remote-proxy.test.ts test/unit/server/coding-cli/codex-app-server/runtime.test.ts --run
• npm run check