persisting credits if new snapshot does not contain credit info (#7490)

in response to incoming changes to responses headers where the header
may sometimes not contain credits info (no longer forcing a credit
check)

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

@@ -18,6 +18,7 @@ use codex_core::protocol::AgentReasoningDeltaEvent;
 use codex_core::protocol::AgentReasoningEvent;
 use codex_core::protocol::ApplyPatchApprovalRequestEvent;
 use codex_core::protocol::BackgroundEventEvent;
+use codex_core::protocol::CreditsSnapshot;
 use codex_core::protocol::Event;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::ExecApprovalRequestEvent;
@@ -521,6 +522,53 @@ fn test_rate_limit_warnings_monthly() {
     );
 }
 
+#[test]
+fn rate_limit_snapshot_keeps_prior_credits_when_missing_from_headers() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual();
+
+    chat.on_rate_limit_snapshot(Some(RateLimitSnapshot {
+        primary: None,
+        secondary: None,
+        credits: Some(CreditsSnapshot {
+            has_credits: true,
+            unlimited: false,
+            balance: Some("17.5".to_string()),
+        }),
+    }));
+    let initial_balance = chat
+        .rate_limit_snapshot
+        .as_ref()
+        .and_then(|snapshot| snapshot.credits.as_ref())
+        .and_then(|credits| credits.balance.as_deref());
+    assert_eq!(initial_balance, Some("17.5"));
+
+    chat.on_rate_limit_snapshot(Some(RateLimitSnapshot {
+        primary: Some(RateLimitWindow {
+            used_percent: 80.0,
+            window_minutes: Some(60),
+            resets_at: Some(123),
+        }),
+        secondary: None,
+        credits: None,
+    }));
+
+    let display = chat
+        .rate_limit_snapshot
+        .as_ref()
+        .expect("rate limits should be cached");
+    let credits = display
+        .credits
+        .as_ref()
+        .expect("credits should persist when headers omit them");
+
+    assert_eq!(credits.balance.as_deref(), Some("17.5"));
+    assert!(!credits.unlimited);
+    assert_eq!(
+        display.primary.as_ref().map(|window| window.used_percent),
+        Some(80.0)
+    );
+}
+
 #[test]
 fn rate_limit_switch_prompt_skips_when_on_lower_cost_model() {
     let (mut chat, _, _) = make_chatwidget_manual();