Function generate 

pub async fn generate(
    provider: &Provider,
    gen: &AuthGenerator,
    ctx: &GenContext,
    keyring: &Keyring,
    cache: &AuthCache,
) -> Result<GeneratedCredential, AuthGenError>

Generate a credential by running the provider’s auth_generator.

1. Check cache → return if hit
2. Expand variables in args and env
3. Spawn subprocess (command or script)
4. Parse output (text or JSON)
5. Cache and return