fix(config): reject object router model refs

[MicroMilo]

Summary

• reject non-string router model references during config validation
• keep router.scenarios.default synced as the runtime-supported provider/model string
• add regression coverage for object-shaped router refs and sync behavior

Verification

• npm --workspace ui exec vitest run server/services/pilotdeckConfig.router-model-ref.test.js
• npm exec -- tsc --noEmit -p tsconfig.json

Notes

This is a real UI/runtime contract mismatch: the runtime accepts provider/model strings, while the previous UI validator silently ignored object-shaped refs.