[BUG] CognitiveService `ConnectionPropertiesV2` variant models are not exclusively declared

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/a4303ecb6929f5bf7fe6238f93920c4f9950e9b4/specification/cognitiveservices/resource-manager/Microsoft.CognitiveServices/stable/2026-05-01/cognitiveservices.json

### API Spec version

2026-05-01

### Describe the bug

In the API definition of Cognitive service, there is a *polymorphic* model called `ConnectionPropertiesV2`, which is discriminated by its `authType` attribute, which in turn has about 20 variant enums. However, not all of these variant corresponds to a specific model.

E.g. `ManagedIdentity` variant has the model declared:

```
"ManagedIdentityAuthTypeConnectionProperties": {
"type": "object",
"properties": {
"credentials": {
"$ref": "#/definitions/ConnectionManagedIdentity"
}
},
"allOf": [
{
"$ref": "#/definitions/ConnectionPropertiesV2"
}
],
"x-ms-discriminator-value": "ManagedIdentity"
},
```

While `AccountManagedIdentity` doesn't have such a specialized model.

This is more or less a grey area by implying *if there is no specialized child model, then fallback to the base model for that variant*. However, *this* undocumented behavior is then leaked to all downstream SDKs/applications/any consumer of the API spec, making it hard to do right. Besides, it introduce a discoverability issue for the SDK users about when shall I use which model, which results into a less ideal UX.

### Expected behavior

Supplement the child model declarations to cover all the variant enums.

### Actual behavior

As is described above: some are covered, some are missing.

### Reproduction Steps

N/A

### Environment

_No response_

Start with the linked specification/cognitiveservices.json file and locate the ConnectionPropertiesV2 definition, its authType discriminator values, and the existing specialized child models. Compare every variant with its declared model, add declarations for the uncovered variants, and validate that the API specification remains consistent.

Written by the indexing model from the issue text.