Function select_provider 

pub async fn select_provider(
    config: &AppConfig,
    secrets: Option<&dyn SecretProvider>,
) -> Box<dyn SttProvider>

Select an STT provider from app configuration.

This is intentionally conservative: if required fields (like API keys or model paths) are missing, it returns an UnconfiguredSttProvider that provides an actionable error message.

If secrets is provided, it will be consulted for API keys using SecretKey fallbacks.