`az snapshot create` from blob uri fails with 2.30

[jkeech]

Describe the bug
az snapshot create fails starting in version 2.30 when using a source that is a blob uri. It works on 2.29. It looks like the change in #19875 to determine the correct copyStart mode doesn't work for the blob uri source mode.

Specifically, it calls result = parse_proxy_resource_id(namespace.source_disk or namespace.source_snapshot) at https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/azure/cli/command_modules/vm/_validators.py#L1643-L1644, but both source_disk and source_snapshot are not set since only source_blob_uri is set.

This results in an error soon after: ERROR: 'resource_group'

To Reproduce
az snapshot create -n {name} -g {group} -l {location} --source {blobUri} --source-storage-account-id {blobStorageAccountId}

Expected behavior
Snapshot creation from a blob uri works

Environment summary
Running in Azure Pipelines. Releases started failing when the agents upgraded to az version 2.30.

Additional context

[jkeech]

Adding --copy-start appears to be a workaround since it avoids the code path with the bug. But my understanding from reading the code is that the --copy-start mode does not even apply to the blob uri source scenario, so it doesn't seem like a great long-term solution to have to specify --copy-start in order to make it work.

[yonzhan]

Compute