AzureAppConfiguration().AsExisting fails with "missing subscription configuration" despite correct env vars and ARM read permissions

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

I am encountering an issue where builder.AddAzureAppConfiguration("config").AsExisting(configName, configResourceGroupName) consistently fails with a 'missing subscription configuration' error, even when all required environment variables (AZURE_SUBSCRIPTION_ID, AZURE_LOCATION) are correctly set and visible to the AppHost process, and the Azure identity has confirmed management plane (ARM) read permissions for the resource.

### Expected Behavior

- The Aspire AppHost should successfully discover and register the existing Azure App Configuration resource by performing an Azure Resource Manager (ARM) lookup.
- The application should start without any errors related to App Configuration discovery, and the App Configuration resource should be correctly integrated into the Aspire application model.

### Steps To Reproduce

1. Create a new .NET Aspire Starter Application (or integrate into an existing one).
2. Add the Microsoft.Net.Aspire.Hosting.Azure NuGet package to the AppHost project.
3. In the Azure subscription, create an Azure App Configuration resource named from an exisitng one within a existing resource group for the known location.
4. Ensure the Azure CLI (or Azure PowerShell) is logged in with an account that has "Owner" or "Contributor" role on the subscription/resource group, or at least "Reader" on the App Configuration resource. (Crucially, confirm this identity has Microsoft.AppConfiguration/configurationStores/read permission).
5. In the Aspire AppHost Program.cs, add the following code:
```csharp
// Program.cs in AppHost project

// ... (any existing usings or builder initialization)

// BEGIN Debugging Output - Confirming Environment Variables
Console.WriteLine("--- AppHost Environment Variables ---");
Console.WriteLine($"AZURE_SUBSCRIPTION_ID: {Environment.GetEnvironmentVariable("AZURE_SUBSCRIPTION_ID")}");
Console.WriteLine($"AZURE_LOCATION: {Environment.GetEnvironmentVariable("AZURE_LOCATION")}");
Console.WriteLine($"DOTNET_ENVIRONMENT: {Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT")}");
Console.WriteLine("--- End AppHost Environment Variables ---");
// END Debugging Output

var configName = builder.AddParameter("name-of-the-appconfiguration");
var configResourceGroupName = builder.AddParameter("name-of-the-resourcegroup");
var appConfig = builder.AddAzureAppConfiguration("config")
.AsExisting(configName, configResourceGroupName);

// ... (rest of your AppHost setup)
```
6. Set the AZURE_SUBSCRIPTION_ID and AZURE_LOCATION environment variables for the AppHost process.
azd env set AZURE_SUBSCRIPTION_ID "c1bcd...04a"
azd env set AZURE_LOCATION "West Europe" # Note exact casing and space

### Exceptions (if any)

Resource could not be provisioned because Azure subscription, location, and resource group information is missing. See https://aka.ms/dotnet/aspire/azure/provisioning for more details.

### .NET Version info

- Aspire Version: 9.3.1
- Target framework: net9.0
- .net SDK version: 9.0.3
- Operation system: Windows 11
- IDE: Microsoft Visual Studio Community 2022 (64-bit) - Version 17.14.7

### Anything else?

- Confirmation the env where correctly set (via console output)
- Confirmation of successful ARM Read Access by running the az resource show --name --resource-group --resource-type Microsoft.AppConfiguration/configurationStores --subscription c1bcd...1d04a