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.

[MicroMilo]

Validation note:

This PR closes a concrete router config serialization drift.

The runtime-supported router model reference contract is a provider/model string, for example:

"main/gpt-test"

Before this change, UI/server validation did not reject object-shaped router refs. In addition, syncAgentModelWithRouter preserved the object-shaped form when the existing default router scenario was already an object, producing a value like:

{ id: "main/gpt-test", provider: "main", model: "gpt-test" }

That object shape is not the runtime contract for router scenario model refs.

The regression test covers both sides of the fix:

object-shaped router.scenarios.default is rejected by validatePilotDeckConfig;
syncAgentModelWithRouter writes back the canonical string form, main/gpt-test.
Validation commands used by this PR:

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

This is a good contract-level fix because it prevents invalid input and also prevents the UI sync path from generating the invalid shape again.