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/rate_limits.rs

@@ -3,6 +3,7 @@ use crate::chatwidget::get_limits_duration;
 use super::helpers::format_reset_timestamp;
 use chrono::DateTime;
 use chrono::Local;
+use chrono::Utc;
 use codex_core::protocol::RateLimitSnapshot;
 use codex_core::protocol::RateLimitWindow;
 
@@ -34,8 +35,7 @@ impl RateLimitWindowDisplay {
     fn from_window(window: &RateLimitWindow, captured_at: DateTime<Local>) -> Self {
         let resets_at = window
             .resets_at
-            .as_deref()
-            .and_then(|value| DateTime::parse_from_rfc3339(value).ok())
+            .and_then(|seconds| DateTime::<Utc>::from_timestamp(seconds, 0))
             .map(|dt| dt.with_timezone(&Local))
             .map(|dt| format_reset_timestamp(dt, captured_at));