claude plugin eval can now use your session's API key

What

claude plugin eval runs tests on a plugin. It now handles an API key (a secret key used instead of signing in) for those test runs.

• Picking a credential: when a key is available and OAuth sign-in is not forced, the key is used. It is also used as a fallback when the OAuth token is missing or cannot be refreshed. Before, the eval path returned nothing when an API key file descriptor or other key source was present.
• Passing it on: when the session's credential is an API key, it is handed to the test runs through CLAUDE_CODE_API_KEY_FILE_DESCRIPTOR.
• Windows: there it stops with "claude plugin eval can't pass this session's API key to its test runs on Windows; set ANTHROPIC_API_KEY and run it again".
• Gateways: gateway credentials still use CLAUDE_CODE_USE_GATEWAY and ANTHROPIC_BASE_URL.

Why

If you run evals with an API key, you no longer have to export the key yourself, except on Windows where you still need to set ANTHROPIC_API_KEY. Which credential your eval runs use may also differ from before.