storing credits (#6858)

Expand the rate-limit cache/TUI: store credit snapshots alongside
primary and secondary windows, render “Credits” when the backend reports
they exist (unlimited vs rounded integer balances)

codex-rs/protocol/src/protocol.rs

@@ -790,6 +790,7 @@ pub struct TokenCountEvent {
 pub struct RateLimitSnapshot {
     pub primary: Option<RateLimitWindow>,
     pub secondary: Option<RateLimitWindow>,
+    pub credits: Option<CreditsSnapshot>,
 }
 
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
@@ -804,6 +805,13 @@ pub struct RateLimitWindow {
     pub resets_at: Option<i64>,
 }
 
+#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
+pub struct CreditsSnapshot {
+    pub has_credits: bool,
+    pub unlimited: bool,
+    pub balance: Option<String>,
+}
+
 // Includes prompts, tools and space to call compact.
 const BASELINE_TOKENS: i64 = 12000;