Remove unused conversation_id header (#9107)

It's an exact copy of session_id

codex-rs/codex-api/src/requests/chat.rs

@@ -393,10 +393,6 @@ mod tests {
             .build(&provider())
             .expect("request");
 
-        assert_eq!(
-            req.headers.get("conversation_id"),
-            Some(&HeaderValue::from_static("conv-1"))
-        );
         assert_eq!(
             req.headers.get("session_id"),
             Some(&HeaderValue::from_static("conv-1"))

codex-rs/codex-api/src/requests/headers.rs

@@ -5,7 +5,6 @@ use http::HeaderValue;
 pub(crate) fn build_conversation_headers(conversation_id: Option<String>) -> HeaderMap {
     let mut headers = HeaderMap::new();
     if let Some(id) = conversation_id {
-        insert_header(&mut headers, "conversation_id", &id);
         insert_header(&mut headers, "session_id", &id);
     }
     headers

codex-rs/codex-api/src/requests/responses.rs

@@ -249,10 +249,6 @@ mod tests {
             .collect();
         assert_eq!(ids, vec![Some("m1".to_string()), None]);
 
-        assert_eq!(
-            request.headers.get("conversation_id"),
-            Some(&HeaderValue::from_static("conv-1"))
-        );
         assert_eq!(
             request.headers.get("session_id"),
             Some(&HeaderValue::from_static("conv-1"))