az vm create --no-wait fails with "Extra data: line 1 column 4 (char 3)"

### Describe the bug

Since yesterday (April 7, 2026), `az vm create --no-wait` fails with:

```
ERROR: Extra data: line 1 column 4 (char 3)
```

This is a JSON parsing error, most likely in the LRO continuation token handling introduced by the pickle-to-JSON migration in `azure-core` 1.38.0.

I can confirm that:
- Azure CLI 2.83.0+ (azure-core >=1.38.0) - fails
- Azure CLI 2.82.0 (azure-core 1.37.0) - works
(tested in GH CI environment, on default Ubuntu runner)

The continuation token format was changed from pickle to JSON in `azure-core` 1.38.0 (https://github.com/Azure/azure-sdk-for-python/pull/44574). A follow-up PR added azure-asyncoperation header handling in azure-mgmt-core (https://github.com/Azure/azure-sdk-for-python/pull/44658).

### Related command

```
az vm create --resource-group $RG --name $VM --nics $NIC --image Ubuntu2404 \
--size Standard_D2s_v5 --admin-username azureuser --ssh-key-values key.pub \
--authentication-type ssh --no-wait
```

### Errors

```
ERROR: Extra data: line 1 column 4 (char 3)
```

### Issue script & Debug output

-

### Expected behavior

Command works.

### Environment Summary

-

### Additional context

The same CLI version (2.84.0, pre-installed on GitHub Actions runner image) worked on April 2 and broke on April 7, pointing to a server-side change exposing the bug.

Pinning `azure-cli` to 2.82.0 (last version using azure-core 1.37.0) helped as a workaround.