Better usage errors (#1941)

<img width="771" height="279" alt="image"
src="https://github.com/user-attachments/assets/e56f967f-bcd7-49f7-8a94-3d88df68b65a"
/>

codex-rs/core/src/error.rs

@@ -62,6 +62,12 @@ pub enum CodexErr {
     #[error("unexpected status {0}: {1}")]
     UnexpectedStatus(StatusCode, String),
 
+    #[error("Usage limit has been reached")]
+    UsageLimitReached,
+
+    #[error("Usage not included with the plan")]
+    UsageNotIncluded,
+
     /// Retry limit exceeded.
     #[error("exceeded retry limit, last status: {0}")]
     RetryLimit(StatusCode),