Get-AzVMBootDiagnosticsData cannot download data for "Managed Storage Account"

### Description of the new feature

If a Windows VM has boot diagnostic setting enabled with the "Managed Storage Account" option, then the command cannot download data and throw the error:

`The specified blob does not exist.`

If you grab the VM's `$VM.DiagnosticsProfile.BootDiagnostics`, the `StorageUri` is empty.

### Proposed implementation details (optional)

_No response_

Start by tracing the Get-AzVMBootDiagnosticsData command and how it reads VM.DiagnosticsProfile.BootDiagnostics.StorageUri. Reproduce the failure with a Windows VM using the Managed Storage Account setting, then verify that boot diagnostics data can be downloaded without relying on a populated StorageUri.

Written by the indexing model from the issue text.