[kusto] az kusto cluster update with --sku specified updates unrelated properties

Source: https://github.com/Azure/azure-cli/issues/33714 (by @mkeesey)
Affected extension: `kusto` (`src/kusto/`)

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# Issue #33714 (by @mkeesey)
## Title
az kusto cluster update with --sku specified updates unrelated properties

## Body
### Describe the bug

When running `az kusto cluster update --name --subscription <sub>--resource-group --sku name="" tier="Standard"` to migrate compute skus, several unrelated properties of the cluster also were updated.</sub>

1. enableAutoStop switched from `false` to `true`
2. enableDiskEncryption switched from `true` to `false`
3. enablePurge switched from `true` to `false`

This is quite unexpected since these properties were not specified, and I do not see anything in the command docs that they are required or have default values.

### Related command

`az kusto cluster update`

### Errors

n/a

### Issue script & Debug output

n/a - appears to reproduce with any invocation that involves a sku migration.

### Expected behavior

I expect the SKU to be updated, but those unconfigured properties to retain their original values.

### Environment Summary

azure-cli 2.87.0 *

core 2.87.0 *
telemetry 1.1.0

Extensions:
acr-transfer Unknown
acrtransfer 1.1.0
aks-preview 18.0.0b43
azure-devops 1.0.2
azure-firewall 1.2.2
kusto 0.5.0
nsp 1.0.0
resource-graph 2.1.0
support 2.0.1

Dependencies:
msal 1.36.0
azure-mgmt-resource 24.0.0

Python location '/bin/python3.12'
Config directory '/home//.azure'
Extensions directory '/home//.azure/cliextensions'
Extensions system directory '/usr/lib/python3.12/site-packages/azure-cli-extensions'

Python (Linux) 3.12.9 (main, Jun 2 2026, 15:06:48) [GCC 13.2.0]

Legal docs and information: aka.ms/AzureCliLegal

### Additional context

_No response_

## Comments
### Comment by @azure-client-tools-bot-prd[bot]
Hi @mkeesey,

2.87.0 is not the latest Azure CLI(2.88.0).

If you haven't already attempted to do so, please upgrade to the latest Azure CLI version by following https://learn.microsoft.com/en-us/cli/azure/update-azure-cli.

### Comment by @mkeesey
Given this would be within the kusto extension, I don't think a minor version bump with the base cli would fix this.
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Start in src/kusto/ and trace the implementation of az kusto cluster update, reproducing a SKU migration to observe how unspecified properties are sent. Check that enableAutoStop, enableDiskEncryption, and enablePurge retain their existing values while the SKU changes; completion should include coverage for this behavior if the relevant tests are found.

Written by the indexing model from the issue text.