Preserve OpenRouter error messages

[xmarquez]

Summary

• Preserve the provider-supplied message when OpenRouter returns an error object.
• Add a snapshot test for this response-error path.

Root cause

check_openrouter_error() extracted error$message, but passed the literal string "message" to abort(). This masked the provider's actual error text.

Validation

• Ran air format on the modified R files.
• Loaded the package and verified that the resulting condition preserves the provider message.

[xmarquez]

Hello @thisisnic - apologies if I needed to raise an issue first; this seemed like a very simple bug I ran into using openrouter for a project (provider error messages were not preserved). Let me know if I need to do anything differently.

[hadley]

Doh.