Web API: re-uploading a downloaded object under another name hangs (duplicate UUID, modal error dialog) #434

Problem

Re-uploading an object downloaded from the Web API under another name hangs the request for 30 seconds and leaves an error dialog open in DataLab:

1. GET /api/v1/objects/{name}/data returns an NPZ archive whose metadata.json holds the object metadata, including its __uuid (WorkspaceAdapter.get_object copies the UUID on purpose).
2. PUT /api/v1/objects/{other}/data with this archive creates a new object carrying the same UUID: ObjectModel.add_objects raises ValueError (UUID already in the model).
3. WorkspaceAdapter._add_object_sync calls panel.add_object, decorated with qt_try_except: the error is shown in a modal dialog on the Qt main thread, the executor waits until its 30-second timeout, and the client gets a read timeout instead of an error response.

This is what WorkspaceBackend.rename of DataLab-Kernel does (get, add under the new name, remove), so its live test test_rename times out against DataLab develop (found while fixing DataLab-Platform/DataLab-Kernel#7). DataLab 1.3 is not affected.

Expected

PUT /objects/{name}/data creates (or replaces) an object: the new object gets its own identity, whatever UUID the archive carries; errors are returned as HTTP errors and never shown as dialogs by the Web API.