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.

[MicroMilo]

Validation note:

This PR closes a real validation/runtime contract gap.

The runtime model parser processes every configured entry under model.providers, not only the provider referenced by agent.model. Before this change, UI/server validation only enforced full provider requirements on the active provider path, so an invalid unused provider could pass /api/config/validate and then fail later during runtime reload/startup.

The regression test captures the minimal failing case:

• agent.model points to a valid main/gpt-test provider/model.
• model.providers.unused is present but has an empty apiKey.
• Validation now fails with model.providers.unused.apiKey is required.

Validation commands used by this PR:

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

This test is directly tied to the workflow contract:
UI config validation -> validatePilotDeckConfig -> runtime model provider parsing.