Function authorize_device_code_flow 

pub async fn authorize_device_code_flow(
    config: &OAuthProviderConfig,
) -> Result<OAuthToken, LlmError>

Run the OAuth 2.0 Device Code flow for headless environments.

1. Request a device code from the provider
2. Display the user code and verification URI
3. Poll the token endpoint until the user completes authorization

Returns the obtained OAuthToken on success.