Propagate session ID when compacting (#12802)

We propagate the session ID when sending requests for inference but we
don't do the same for compaction requests. This makes it hard to link
compaction requests to their session for debugging purposes

codex-rs/core/src/client.rs

@@ -315,7 +315,10 @@ impl ModelClient {
             instructions: &instructions,
         };
 
-        let extra_headers = self.build_subagent_headers();
+        let mut extra_headers = self.build_subagent_headers();
+        extra_headers.extend(build_conversation_headers(Some(
+            self.state.conversation_id.to_string(),
+        )));
         client
             .compact_input(&payload, extra_headers)
             .await