[BUG] DataProtection@2025-09-01 - `BackupVaults_CreateOrUpdate` fails to respond with accepted content

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/dataprotection/resource-manager/Microsoft.DataProtection/DataProtection/stable/2025-09-01/dataprotection.json#L570

### API Spec version

2025-09-01

### Describe the bug

The API responds with a 406 when the `Accept` header includes a charset, e.g. `Accept: application/json; charset=utf-8`.

This used to work fine, as this has been a part of the requests sent by the AzureRM provider for a long time ~~but as of a couple days ago~~ this is now the response on failed requests (e.g. 400s)

Edit: after more investigation this has been an issue for longer than a few days

Encountered on `2025-09-01` however it appears this behaviour is consistent across all API versions (tested up until `2023-01-01`

### Expected behavior

API produces a response matching the accepted content

### Actual behavior

406 status code

### Reproduction Steps

Create a backup vault with an invalid value for one of the properties (`properties.securitySettings.ImmutabilitySettings.State` in this example) and ensure the request contains an `Accept` header specifying `charset=utf-8` (or any other, the type doesn't appear to matter)

```
PUT https://management.azure.com/subscriptions/{subscriptionID}/resourceGroups/{resourceGroupName}/providers/Microsoft.DataProtection/backupVaults/{backupVaultName}?api-version=2025-09-01

{
"identity": {
"type": "None"
},
"location": "westeurope",
"properties": {
"storageSettings": [{
"type": "LocallyRedundant",
"datastoreType": "VaultStore"
}],
"securitySettings": {
"immutabilitySettings": {
"state": "Foo"
}
}
},
"tags": {}
}
```

### Environment

Terraform

Start with the DataProtection API specification at specification/dataprotection/resource-manager/Microsoft.DataProtection/DataProtection/stable/2025-09-01/dataprotection.json#L570 and review the BackupVaults_CreateOrUpdate definition. Reproduce the PUT request using the supplied invalid immutability state and Accept header, then compare the documented response contract with the observed 406. Done means identifying the appropriate specification change or confirming that the behavior must be fixed outside this repository.

Written by the indexing model from the issue text.