[codex] suppress low usage remaining warnings when credits are available (#28593)

## Why

The TUI computed proactive `Heads up, you have less than ...` warnings
before considering workspace credits. As a result, users could see
included-limit warnings even when they could continue using Codex with
workspace credits.

`has_credits` alone is not sufficient to determine whether finite
credits are usable: a spend-control hard limit can cap the reported
balance to zero while `has_credits` still reflects the workspace's raw
balance. Unlimited credits are the opposite case: they are usable even
though no numeric balance is reported.

## What changed

- suppress proactive TUI rate-limit usage warnings and the lower-cost
model nudge when usable workspace credits are available
- treat credits as usable when `has_credits` is true and either
`unlimited` is true or the parsed balance is positive
- continue showing warnings when the usable balance is zero, including
when a spend-control limit has capped otherwise available workspace
credits
- add regression coverage for zero-balance, positive-balance, and
unlimited workspace-credit snapshots

## Validation

- `just test -p codex-tui rate_limit_usage_warnings_`

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

@@ -210,7 +210,19 @@ impl ChatWidget {
             {
                 self.codex_rate_limit_reached_type = Some(rate_limit_reached_type);
             }
-            let warnings = if is_codex_limit {
+
+            let has_workspace_credits = snapshot.credits.as_ref().is_some_and(|credits| {
+                credits.has_credits
+                    && (credits.unlimited
+                        || credits.balance.as_deref().is_some_and(|balance| {
+                            balance
+                                .trim()
+                                .parse::<f64>()
+                                .is_ok_and(|balance| balance > 0.0)
+                        }))
+            });
+            let should_warn_about_rate_limit_usage = is_codex_limit && !has_workspace_credits;
+            let warnings = if should_warn_about_rate_limit_usage {
                 self.rate_limit_warnings.take_warnings(
                     snapshot
                         .secondary
@@ -245,12 +257,6 @@ impl ChatWidget {
                         .map(|w| f64::from(w.used_percent) >= RATE_LIMIT_SWITCH_PROMPT_THRESHOLD)
                         .unwrap_or(false));
 
-            let has_workspace_credits = snapshot
-                .credits
-                .as_ref()
-                .map(|credits| credits.has_credits)
-                .unwrap_or(false);
-
             if high_usage
                 && !has_workspace_credits
                 && !self.rate_limit_switch_prompt_hidden()

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

@@ -1009,6 +1009,82 @@ async fn rate_limit_switch_prompt_skips_non_codex_limit() {
     ));
 }
 
+#[tokio::test]
+async fn rate_limit_usage_warnings_show_when_workspace_credits_zero_balance() {
+    let (mut chat, mut rx, _) = make_chatwidget_manual(Some("gpt-5")).await;
+    chat.has_chatgpt_account = true;
+
+    let mut rate_limit_snapshot = snapshot(/*percent*/ 95.0);
+    rate_limit_snapshot.credits = Some(CreditsSnapshot {
+        has_credits: true,
+        unlimited: false,
+        balance: Some("0".to_string()),
+    });
+
+    chat.on_rate_limit_snapshot(Some(rate_limit_snapshot));
+
+    assert!(
+        !drain_insert_history(&mut rx).is_empty(),
+        "zero-balance workspace credits should not suppress proactive usage warnings"
+    );
+    assert!(matches!(
+        chat.rate_limit_switch_prompt,
+        RateLimitSwitchPromptState::Pending
+    ));
+}
+
+#[tokio::test]
+async fn rate_limit_usage_warnings_skip_when_workspace_credits_are_available() {
+    let (mut chat, mut rx, _) = make_chatwidget_manual(Some("gpt-5")).await;
+    chat.has_chatgpt_account = true;
+
+    let mut rate_limit_snapshot = snapshot(/*percent*/ 95.0);
+    rate_limit_snapshot.credits = Some(CreditsSnapshot {
+        has_credits: true,
+        unlimited: false,
+        balance: Some("25.00".to_string()),
+    });
+
+    chat.on_rate_limit_snapshot(Some(rate_limit_snapshot));
+
+    assert!(
+        drain_insert_history(&mut rx).is_empty(),
+        "workspace credits should suppress proactive usage warnings"
+    );
+    assert!(matches!(
+        chat.rate_limit_switch_prompt,
+        RateLimitSwitchPromptState::Idle
+    ));
+    assert_eq!(
+        chat.rate_limit_warnings.primary_index, 0,
+        "suppressed warnings should not consume warning thresholds"
+    );
+}
+
+#[tokio::test]
+async fn rate_limit_usage_warnings_skip_with_unlimited_workspace_credits() {
+    let (mut chat, mut rx, _) = make_chatwidget_manual(Some("gpt-5")).await;
+    chat.has_chatgpt_account = true;
+
+    let mut rate_limit_snapshot = snapshot(/*percent*/ 95.0);
+    rate_limit_snapshot.credits = Some(CreditsSnapshot {
+        has_credits: true,
+        unlimited: true,
+        balance: None,
+    });
+
+    chat.on_rate_limit_snapshot(Some(rate_limit_snapshot));
+
+    assert!(
+        drain_insert_history(&mut rx).is_empty(),
+        "unlimited workspace credits should suppress proactive usage warnings"
+    );
+    assert!(matches!(
+        chat.rate_limit_switch_prompt,
+        RateLimitSwitchPromptState::Idle
+    ));
+}
+
 #[tokio::test]
 async fn rate_limit_switch_prompt_shows_once_per_session() {
     let (mut chat, _, _) = make_chatwidget_manual(Some("gpt-5")).await;