fix: keep scoped models registry additive to models.dev

[rexdotsh]

Summary

• keep scoped discovery responses additive to the upstream models.dev registry instead of replacing it with scoped proxy providers
• keep provider and account scopes limited to the appended kleis aggregate models while leaving upstream provider entries unchanged
• remove the now-unused scoped canonical provider patching path and update registry contract coverage

Testing

• bun test
• bun typecheck