* fix(coding-agent): surface models.json load errors on stderr in --list-models
When models.json has validation errors, --list-models silently
discarded custom models and overrides without any user-visible
feedback. Now prints the error to stderr via chalk.yellow warning
before listing available models.
* fix(coding-agent): allow custom models for built-in providers in models.json
Built-in providers (openrouter, anthropic, etc.) already have api and
baseUrl on every model, and auth comes from env vars / auth storage.
Relax validation so custom models under built-in providers don't need
redundant baseUrl, apiKey, or api fields. Inherit them from the first
built-in model for that provider.
fixes#2921
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Co-authored-by: Mario Zechner <badlogicgames@gmail.com>
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