Unable to retrieve value for secret in already provisioned Key Vault

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

Attempting to read a value from an existing Key Vault secret in an AppHost with the following code:

```csharp
var builder = DistributedApplication.CreateBuilder(args);

var keyVaultNameParam = builder.AddParameter("integration-keyvault-name");
var keyVaultResourceGroupParam = builder.AddParameter("integration-keyvault-resource-group-name");

var integrationVault = builder.AddAzureKeyVault("integration-keyvault")
.AsExisting(keyVaultNameParam, keyVaultResourceGroupParam);

var connectionString = integrationVault.GetSecret("MyConnectionString");

var connectionStringValue = await connectionString.GetValueAsync();
```

Throws:

```
System.InvalidOperationException: 'Secret 'MyConnectionString' not found in Key Vault 'integration-keyvault'.'
```

I've been through the Aspire docs and unable to find anything else that needs to be configured for this to work. What am I missing?

appsettings.json

```json

{
"Azure": {
"SubscriptionId": "",
"AllowResourceGroupCreation": false,
"ResourceGroup": "",
"CredentialSource": "VisualStudio"
},
"Parameters": {
"integration-keyvault-name": "",
"integration-keyvault-resource-group-name": ""
}
}
```

### Expected Behavior

_No response_

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version info

_No response_

### Anything else?

dotnet 8.0
Visual Studio 22 17.14.20
Aspire.Hosting.Apphost 13.0.2
Aspire.Hosting.Azure 13.0.2
Aspire.Hosting.Azure.KeyVault 13.0.2