Cloud Requirements: increase timeout and retries (#10631)

Add retries and an increased-length timeout for loading Cloud
Requirements.

Co-authored-by: alexsong-oai <alexsong@openai.com>

codex-rs/core/src/util.rs

@@ -37,7 +37,7 @@ macro_rules! feedback_tags {
     };
 }
 
-pub(crate) fn backoff(attempt: u64) -> Duration {
+pub fn backoff(attempt: u64) -> Duration {
     let exp = BACKOFF_FACTOR.powi(attempt.saturating_sub(1) as i32);
     let base = (INITIAL_DELAY_MS as f64 * exp) as u64;
     let jitter = rand::rng().random_range(0.9..1.1);