The previous nested folder upload flow bypassed the backend batch-upload
contract when parentId was set. Instead of creating the whole metadata
tree in one backend operation, the frontend recursively called
createFolder/getFileUploadUrl for each node.
That introduced two regressions for uploads into subfolders:
- consistency regression: mid-sequence failures could leave partially
created folder trees under the destination folder
- performance regression: metadata creation degraded from a single
batch request to O(files + folders) round-trips before file bytes
were uploaded
This change moves nested uploads back to the original batch semantics:
- add optional parent_id support to app asset batch-upload payload
- create the whole nested tree under the target parent in
AppAssetService.batch_create_from_tree
- pass parentId through useBatchUpload instead of using per-node
createFolder/getFileUploadUrl calls
- remove the now-unnecessary useBatchUploadOperation wrapper
- add a backend unit test covering batch tree creation under an
existing parent folder
After this change, both root uploads and subfolder uploads use the same
single-request metadata creation path, preserving atomic tree creation
semantics and avoiding avoidable metadata round-trips.
