fix: keep provider-returned models visible

[DhyeyMavani2003]

Addressing #8321

Summary

• stop treating the canonical registry as an allowlist in fetch_recommended_models
• keep every provider-returned model visible while still ranking canonically-known text/tool-capable models first
• add regression coverage for unknown models and custom proxy model lists

Testing

• cargo fmt

Context

This addresses the provider model list behavior where Goose silently hid models that were returned by the provider but were missing from the bundled canonical registry. The registry is now advisory for ranking instead of gating visibility.