ca93d2f0fe
When model cache is empty, the fallback chain resolution was blindly trusting connected providers without verifying if the model actually exists. This caused errors when a provider (e.g., opencode) was marked as connected but didn't have the requested model (e.g., claude-haiku-4-5). Now skips fallback chain entirely when model cache is unavailable and falls through to system default, letting OpenCode handle the resolution.