refactor(coprocessor): remove last non-cross-boundary set_parent, document remaining

- Replace set_parent(t.context()) with parent: &t in batch_store span
  (same execution context, not cross-boundary)
- Add comments on the two remaining set_parent calls explaining they
  restore OTel context across async/thread boundaries

Author: Eikix

coprocessor/fhevm-engine/sns-worker/src/aws_upload.rs

@@ -164,6 +164,8 @@ async fn run_uploader_loop(
 
                 // Spawn a new task to upload the ciphertexts
                 let h = tokio::spawn(async move {
+                    // Cross-boundary: spawned task; restore the OTel context
+                    // that was captured when the upload item was created.
                     let upload_span = error_span!("upload_s3", operation = "upload_s3");
                     upload_span.set_parent(item.otel.context());
                     match upload_ciphertexts(trx, item, &client, &conf)

coprocessor/fhevm-engine/sns-worker/src/executor.rs

@@ -395,10 +395,10 @@ async fn fetch_and_execute_sns_tasks(
         update_computations_status(trx, &tasks).await?;
 
         let batch_store_span = tracing::info_span!(
+            parent: &t,
             "batch_store_ciphertext128",
             operation = "batch_store_ciphertext128"
         );
-        batch_store_span.set_parent(t.context());
         let batch_store = async {
             update_ciphertext128(trx, &tasks).await?;
             notify_ciphertext128_ready(trx, &conf.db.notify_channel).await?;
@@ -569,6 +569,8 @@ fn compute_task(
 ) {
     let started_at = SystemTime::now();
     let thread_id = format!("{:?}", std::thread::current().id());
+    // Cross-boundary: compute_task runs on a thread-pool worker;
+    // restore the OTel context that was captured when the task was enqueued.
     let span = error_span!("compute", thread_id = %thread_id);
     span.set_parent(task.otel.context());
     let _enter = span.enter();