[BUG] Role Assignment API returns 500 instead of validation error for invalid principalId

### API Spec link

https://learn.microsoft.com/en-us/rest/api/authorization/role-assignments/create?tabs=HTTP

### API Spec version

2022-04-01

### Describe the bug

When creating a role assignment for an Azure AD group, the `properties.principalId` should be the object ID (GUID) of the AD group.

If an **invalid GUID** (non-existent principalId) is passed:
- The API returns **500 Internal Server Error** with a generic message.
- Instead, it should return a **descriptive 4xx error** (e.g., 400 BadRequest or 404 NotFound).

---

### Expected behavior
Return an error message clearly stating that the provided `principalId` is invalid, e.g.:

```json
{
"error": {
"code": "InvalidPrincipalId",
"message": "The specified principalId is not valid or does not exist."
}
}

### Expected behavior

{
"error": {
"code": "InvalidPrincipalId",
"message": "The specified principalId is not valid or does not exist."
}
}

### Actual behavior

{
"error": {
"code": "InternalServerError",
"message": "Encountered internal server error. diagnostic information..."
}
}

### Reproduction Steps

# Login
az login
$subscriptionId = ""
$resourceGroup = ""
$scope = "/subscriptions/$subscriptionId/resourceGroups/$resourceGroup"

$roleDefinitionId = "/subscriptions/$subscriptionId/providers/Microsoft.Authorization/roleDefinitions/"

$body = @{
properties = @{
principalId = "00000000-0000-0000-0000-000000000000" # Invalid objectId
roleDefinitionId = $roleDefinitionId
scope = $scope
}
} | ConvertTo-Json -Depth 3

$uri = "https://management.azure.com$scope/providers/Microsoft.Authorization/roleAssignments/$(New-Guid)?api-version=2022-04-01"
Invoke-RestMethod -Uri $uri -Method Put -Headers @{Authorization = "Bearer $((az account get-access-token --query accessToken -o tsv))"} -Body $body -ContentType "application/json"

### Environment

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### 🔹 5. Click **Create**
Hit the green **Create** button at the bottom.

---

✅ Done! Your bug is now officially reported to Microsoft.

Do you want me to also give you a **short version** (just 3–4 lines) that you can submit on [Azure Feedback](https://feedback.azure.com/) for extra visibility?

No repository files or tests are named. Start by reviewing the linked Azure Role Assignments create specification and running the PowerShell reproduction with the invalid principalId; done would require the service or specification to produce a descriptive 4xx validation error instead of a generic 500 response.

Written by the indexing model from the issue text.