Fix race condition in ParameterUsingWithTests causing flaky test failures (#19317)

`ParameterUsingWithTests` had a shared `static readonly ServiceBuilder`
field that parallel tests mutated concurrently via `WithMockFileSystem`,
causing intermittent `System.ArgumentException: Destination array was
not long enough`.

## Change

Convert `Services` from a shared static field to a property so each test
gets an independent `ServiceBuilder` instance:

```csharp
// Before — shared, mutable across parallel tests
private static readonly ServiceBuilder Services = new ServiceBuilder()
    .WithFeatureOverrides(new(DeployCommandsEnabled: true));

// After — fresh instance per test
private static ServiceBuilder Services => new ServiceBuilder()
    .WithFeatureOverrides(new(DeployCommandsEnabled: true));
```

This matches the pattern used throughout the rest of the test suite.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: brendandburns <5751682+brendandburns@users.noreply.github.com>

Author: Copilot

src/Bicep.Core.IntegrationTests/ParameterUsingWithTests.cs

@@ -20,7 +20,7 @@ namespace Bicep.Core.IntegrationTests;
 [TestClass]
 public class ParameterUsingWithTests
 {
-    private static readonly ServiceBuilder Services = new ServiceBuilder()
+    private static ServiceBuilder Services => new ServiceBuilder()
         .WithFeatureOverrides(new(DeployCommandsEnabled: true));
 
     [NotNull]