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/core/src/codex.rs

@@ -2481,7 +2481,10 @@ mod tests {
     use crate::tools::format_exec_output_str;
 
     use crate::protocol::CompactedItem;
+    use crate::protocol::CreditsSnapshot;
     use crate::protocol::InitialHistory;
+    use crate::protocol::RateLimitSnapshot;
+    use crate::protocol::RateLimitWindow;
     use crate::protocol::ResumedHistory;
     use crate::state::TaskKind;
     use crate::tasks::SessionTask;
@@ -2551,6 +2554,75 @@ mod tests {
         assert_eq!(expected, actual);
     }
 
+    #[test]
+    fn set_rate_limits_retains_previous_credits() {
+        let codex_home = tempfile::tempdir().expect("create temp dir");
+        let config = Config::load_from_base_config_with_overrides(
+            ConfigToml::default(),
+            ConfigOverrides::default(),
+            codex_home.path().to_path_buf(),
+        )
+        .expect("load default test config");
+        let config = Arc::new(config);
+        let session_configuration = SessionConfiguration {
+            provider: config.model_provider.clone(),
+            model: config.model.clone(),
+            model_reasoning_effort: config.model_reasoning_effort,
+            model_reasoning_summary: config.model_reasoning_summary,
+            developer_instructions: config.developer_instructions.clone(),
+            user_instructions: config.user_instructions.clone(),
+            base_instructions: config.base_instructions.clone(),
+            compact_prompt: config.compact_prompt.clone(),
+            approval_policy: config.approval_policy,
+            sandbox_policy: config.sandbox_policy.clone(),
+            cwd: config.cwd.clone(),
+            original_config_do_not_use: Arc::clone(&config),
+            features: Features::default(),
+            exec_policy: Arc::new(ExecPolicy::empty()),
+            session_source: SessionSource::Exec,
+        };
+
+        let mut state = SessionState::new(session_configuration);
+        let initial = RateLimitSnapshot {
+            primary: Some(RateLimitWindow {
+                used_percent: 10.0,
+                window_minutes: Some(15),
+                resets_at: Some(1_700),
+            }),
+            secondary: None,
+            credits: Some(CreditsSnapshot {
+                has_credits: true,
+                unlimited: false,
+                balance: Some("10.00".to_string()),
+            }),
+        };
+        state.set_rate_limits(initial.clone());
+
+        let update = RateLimitSnapshot {
+            primary: Some(RateLimitWindow {
+                used_percent: 40.0,
+                window_minutes: Some(30),
+                resets_at: Some(1_800),
+            }),
+            secondary: Some(RateLimitWindow {
+                used_percent: 5.0,
+                window_minutes: Some(60),
+                resets_at: Some(1_900),
+            }),
+            credits: None,
+        };
+        state.set_rate_limits(update.clone());
+
+        assert_eq!(
+            state.latest_rate_limits,
+            Some(RateLimitSnapshot {
+                primary: update.primary.clone(),
+                secondary: update.secondary,
+                credits: initial.credits,
+            })
+        );
+    }
+
     #[test]
     fn prefers_structured_content_when_present() {
         let ctr = CallToolResult {