Function is_retryable_error 

pub fn is_retryable_error(
    error_message: &str,
    usage_input_tokens: Option<u64>,
    context_window: Option<u32>,
) -> bool

Check whether an error is retryable (transient). Matches pi-mono’s _isRetryableError() logic:

1. Error message must be non-empty.
2. Must NOT be context overflow (those need compaction, not retry).
3. Must match a retryable pattern (rate limit, server error, etc.).