Use int timestamps for rate limit reset_at (#5383)

The backend will be returning unix timestamps (seconds since epoch)
instead of RFC 3339 strings. This will make it more ergonomic for
developers to integrate against - no string parsing.

codex-rs/tui/src/status/tests.rs

@@ -62,10 +62,10 @@ fn sanitize_directory(lines: Vec<String>) -> Vec<String> {
         .collect()
 }
 
-fn reset_at_from(captured_at: &chrono::DateTime<chrono::Local>, seconds: i64) -> String {
+fn reset_at_from(captured_at: &chrono::DateTime<chrono::Local>, seconds: i64) -> i64 {
     (*captured_at + ChronoDuration::seconds(seconds))
         .with_timezone(&Utc)
-        .to_rfc3339()
+        .timestamp()
 }
 
 #[test]