[codex] fix codex app-server stdio transport

[alanJae]

Summary

• default the main Codex app-server client to stdio:// transport instead of ws://127.0.0.1:<port>
• keep CODEX_APP_SERVER_TRANSPORT=websocket as an explicit fallback for older Codex builds
• update the paused packages/codex-native-api client to use stdio:// as well
• update the Codex app client spawn tests for the new default transport

Root Cause

Newer Codex App CLI builds, verified with codex-cli 0.131.0-alpha.9, reject WebSocket listen URLs:

unsupported --listen URL `ws://127.0.0.1:<port>`; expected `stdio://`, `unix://`, `unix://PATH`, or `off`

CodexBridge still launched codex app-server --listen ws://127.0.0.1:<port>, causing normal WeChat turns to fail before the app-server opened.

Validation

• npm test -- test/providers/codex/app_client.test.ts
• npm run typecheck
• npm run codex-native-api:typecheck
• real local smoke against /Applications/Codex.app/Contents/Resources/codex using CodexAppClient.start() + listThreads({ limit: 1 })