feat: expose FileUploadSession low-level functions (#855)

xet_data/src/processing/file_upload_session.rs

@@ -611,7 +611,8 @@ impl FileUploadSession {
     /// Register a pre-composed file reconstruction plan (MDBFileInfo) with this session.
     /// Used for append-aware writes where the caller builds the reconstruction plan
     /// from existing segments + newly uploaded segments.
-    pub(crate) async fn register_composed_file(self: &Arc<Self>, file_info: MDBFileInfo) -> Result<()> {
+    pub async fn register_composed_file(self: &Arc<Self>, file_info: MDBFileInfo) -> Result<()> {
+        self.check_not_finalized()?;
         self.shard_interface.add_file_reconstruction_info(file_info).await
     }
 
@@ -622,6 +623,10 @@ impl FileUploadSession {
         Ok(())
     }
 
+    pub fn client(&self) -> Arc<dyn Client + Send + Sync> {
+        Arc::clone(&self.client)
+    }
+
     pub fn progress(&self) -> &Arc<GroupProgress> {
         &self.progress
     }