az connectedk8s connect --distribution does not override auto-detected value; blocks Azure Policy extension troubleshooting

### Describe the bug

The `--distribution` flag provided to az connectedk8s connect does not override the auto-detected Kubernetes distribution when a known distribution (e.g. `kubeadm`) is detected.

Even when `--distribution generic` is explicitly specified, the connected cluster resource always persists the detected value (`kubeadm`). There is no supported way to opt out of distribution auto-detection or force the user-specified value.

This behavior is not clearly documented and has practical impact when troubleshooting extension installation issues.

### Related command

1. Use a **kubeadm**-based cluster (Tanzu / TKG / Cluster API on vSphere)
2. Run:
```
az connectedk8s connect \
--name arc-sandbox \
--resource-group rg-sandbox-arc \
--kube-config ~/.kube/config \
--kube-context \
--distribution generic \
--location westcentralus \
--debug
```
3. Query the connected cluster:
```
az connectedk8s show \
--name arc-sandbox \
--resource-group rg-sandbox-arc \
--query distribution
```

### Errors

```
(ExtensionOperationFailed)
Failed to resolve the extension version from the given values.

InnerError:
Internal error: Extension type microsoft.policyinsights doesn't have any supporting artifacts.
```

### Status payload:

```
{
"code": "InstallationFailed",
"displayStatus": null,
"level": null,
"message": "Failed to resolve the extension version from the given values. InnerError: 'Internal error: Extension type microsoft.policyinsights doesn't have any supporting artifacts. Please contact Microsoft support for further assistance.'. Please refer https://aka.ms/k8s-extension-type-versions to find the correct version for your installation . For general troubleshooting visit: https://aka.ms/k8s-extensions-TSG ",
"time": null
}
```

### Issue script & Debug output

From the logs:

`Step: Determining Cluster Distribution and Infrastructure`
`Step: Azure resource provisioning has begun`
```
Request body:
cli.azure.cli.core.sdk.policies: {"location": "westcentralus", "identity": {"type": "SystemAssigned"}, "properties": {"agentPublicKeyCertificate": "...", "distribution": "kubeadm", "infrastructure": "vsphere", "oidcIssuerProfile": {"enabled": true}, "securityProfile": {"workloadIdentity": {"enabled": true}}}, "tags": {}}
```

### Expected behavior

When --distribution generic is explicitly provided, one of the following should occur:

- The connected cluster resource persists: `"distribution": "generic"`
- OR the CLI clearly documents and/or warns that --distribution cannot override auto-detection.

### Actual Behavior
The connected cluster resource always shows: `"distribution": "kubeadm"`

Auto-detection overrides the user-provided value every time.

### Environment Summary

**Azure CLI version**: 2.86.0
**connectedk8s extension version**: 1.11.0
**Cluster type**: kubeadm-based (Tanzu / Cluster API)
**Infrastructure**: vSphere
**Arc agent version**: 1.34.2
**Location**: westcentralus

### Additional context

When the cluster is classified as distribution = kubeadm, installing the Azure Policy extension fails.

### Connected cluster details
```
{
"distribution": "kubeadm",
"infrastructure": "vsphere",
"kubernetesVersion": "1.30.1+vmware.1-fips",
"agentVersion": "1.34.2",
"location": "westcentralus"
}
```

az k8s-extension create \
--cluster-name arc-sandbox \
--cluster-type connectedClusters \
--resource-group rg-sandbox-arc \
--extension-type Microsoft.PolicyInsights \
--name azurepolicy

Reproduce the issue with the documented az connectedk8s connect command using --distribution generic, then run az connectedk8s show --query distribution and inspect the debug request body. Trace where distribution is auto-detected and where the connected cluster request is assembled. Done means the explicit value is persisted as generic, or the CLI clearly warns and documents that auto-detection cannot be overridden.

Written by the indexing model from the issue text.