fix(app-assets): restore atomic batch upload for nested folder targets

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.

api/tests/unit_tests/services/test_app_asset_service.py

@@ -0,0 +1,86 @@
+from types import SimpleNamespace
+
+from core.app.entities.app_asset_entities import AppAssetFileTree, AppAssetNode, AssetNodeType, BatchUploadNode
+from core.app_assets.storage import AssetPaths
+from services import app_asset_service
+from services.app_asset_service import AppAssetService
+
+
+class DummyLock:
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc, tb):
+        return False
+
+
+class DummySession:
+    committed: bool
+
+    def __init__(self) -> None:
+        self.committed = False
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc, tb):
+        return False
+
+    def commit(self) -> None:
+        self.committed = True
+
+
+class DummyStorage:
+    keys: list[str]
+
+    def __init__(self) -> None:
+        self.keys = []
+
+    def get_upload_url(self, key: str, expires_in: int) -> str:
+        self.keys.append(key)
+        return f"https://upload.local/{key}?expires_in={expires_in}"
+
+
+def test_batch_create_from_tree_creates_nodes_under_parent(monkeypatch):
+    session = DummySession()
+    storage = DummyStorage()
+    tenant_id = "11111111-1111-4111-8111-111111111111"
+    app_id = "22222222-2222-4222-8222-222222222222"
+    parent_folder = AppAssetNode.create_folder("33333333-3333-4333-8333-333333333333", "existing-parent")
+    assets = SimpleNamespace(asset_tree=AppAssetFileTree(nodes=[parent_folder]), updated_by=None)
+    app_model = SimpleNamespace(id=app_id, tenant_id=tenant_id)
+
+    monkeypatch.setattr(AppAssetService, "_lock", staticmethod(lambda _app_id: DummyLock()))
+    monkeypatch.setattr(app_asset_service, "db", SimpleNamespace(engine=object()))
+    monkeypatch.setattr(app_asset_service, "Session", lambda *args, **kwargs: session)
+    monkeypatch.setattr(
+        AppAssetService,
+        "get_or_create_assets",
+        staticmethod(lambda _session, _app_model, _account_id: assets),
+    )
+    monkeypatch.setattr(AppAssetService, "get_storage", staticmethod(lambda: storage))
+
+    result = AppAssetService.batch_create_from_tree(
+        app_model,
+        "account-1",
+        [
+            BatchUploadNode(
+                name="docs",
+                node_type=AssetNodeType.FOLDER,
+                children=[
+                    BatchUploadNode(name="guide.md", node_type=AssetNodeType.FILE, size=12),
+                ],
+            ),
+        ],
+        parent_id=parent_folder.id,
+    )
+
+    created_folder = next(node for node in assets.asset_tree.nodes if node.name == "docs")
+    created_file = next(node for node in assets.asset_tree.nodes if node.name == "guide.md")
+
+    assert created_folder.parent_id == parent_folder.id
+    assert created_file.parent_id == created_folder.id
+    assert created_file.id == result[0].children[0].id
+    assert assets.updated_by == "account-1"
+    assert session.committed is True
+    assert storage.keys == [AssetPaths.draft(tenant_id, app_id, created_file.id)]