Add URL to responses error messages (#8984)

Put the URL in error messages, to aid debugging Codex pointing at wrong
endpoints.

<img width="759" height="164" alt="Screenshot 2026-01-09 at 16 32 49"
src="https://github.com/user-attachments/assets/77a0622c-955d-426d-86bb-c035210a4ecc"
/>

codex-rs/codex-client/src/error.rs

@@ -7,6 +7,7 @@ pub enum TransportError {
     #[error("http {status}: {body:?}")]
     Http {
         status: StatusCode,
+        url: Option<String>,
         headers: Option<HeaderMap>,
         body: Option<String>,
     },