When opencode-go API key is set, it should appear in the /model list.
The provider was already in PROVIDER_TO_MODELS_DEV and PROVIDER_REGISTRY,
so it appears via Part 1 (built-in source).
Also fixes a potential issue in Part 2 (HERMES_OVERLAYS) where providers
with auth_type=api_key but no extra_env_vars would not be detected:
- Now also checks api_key_env_vars from PROVIDER_REGISTRY for api_key auth_type
- Add test verifying opencode-go appears when OPENCODE_GO_API_KEY is set