fix: make model configuration resilient

[huyua9]

Problem

ai config ui currently fails before showing its menu when OPENAI_KEY is not configured. Selecting Model also assumes that every OpenAI-compatible endpoint exposes a usable /models response, so custom endpoints can leave the user with an empty selector or an exception.

What changed

• Allow the configuration UI and ai config get to read config without requiring an API key.
• Skip /models lookup when no key is configured and fall back to manual model-id entry.
• Fall back to manual entry when /models fails or returns no usable model IDs.
• Keep a Custom model entry available even when a model list is returned.
• Filter unusable model records before rendering the selector.

Validation

• npm run typecheck
• npm run build
• ./node_modules/.bin/jiti tests/config-model-fallback.test.ts
• Prettier check
• ESLint (with the repository's existing no-var-requires baseline error disabled for validation)
• git diff --check