fix(config): validate all model providers

[MicroMilo]

Summary

• validate the base shape of every configured model provider, not only the active agent provider
• keep active-provider protocol/url requirements on the active path
• add a focused server-side regression test for an unused provider missing apiKey

Verification

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

Notes

This is a real config/runtime mismatch: the UI save path previously allowed invalid unused providers, while the runtime parses all providers during reload.