Add InstantHandle annotations for NUnit Assert.DoesNotThrow overloads

### Summary

JetBrains' NUnit external annotations do not currently cover the `System.Action` overloads introduced by NUnit 4.6 for `Assert.DoesNotThrow`. As a result, Rider/ReSharper inspections can treat the callback as escaping the current scope and report false-positive closure/disposal warnings.

### Affected API

NUnit 4.6.0 and later add these overloads:

```csharp
public static void DoesNotThrow(Action code);
public static void DoesNotThrow(Action code, string message, params object[] args);
```

The existing JetBrains annotations cover the older `NUnit.Framework.TestDelegate` overloads, but not these `System.Action` overloads:

```xml

```

### How to reproduce

With NUnit 4.6.1 and Rider/ReSharper's nullable/disposal inspections enabled:

```csharp
using NUnit.Framework;
using System.Windows.Forms;

using var treeView = new TreeView();

Assert.DoesNotThrow(() => treeView.Nodes.Clear());
```

The callback is executed synchronously by NUnit, but the `Action` overload is not recognized as an instant-handle callback because the external annotation is missing. This results in a false-positive warning about using a disposable value from an outer scope ("AccessToDisposedClosure: Captured variable is disposed in the outer scope").

### Proposed change

The `code` parameter of both `System.Action` overloads should be annotated with `JetBrains.Annotations.InstantHandleAttribute`, just like the existing `TestDelegate` overloads. Rider/ReSharper should then understand that the callback is invoked immediately and should not report the false positive.

### Proposed annotation

```xml

```

### Version information

- NUnit: 4.6.1 (introduced with the NUnit 4.6 API changes)
- JetBrains Rider: current 2026.2 installation tested
- Target framework: .NET Framework 4.6.2

### Related references

- NUnit PR [#5196](https://github.com/nunit/nunit/pull/5196), which introduced the delegate API changes

### Workaround

Projects can temporarily provide the proposed XML in an `ExternalAnnotations/NUnit.Framework/` folder next to their solution. Alternatively, they can avoid the `DoesNotThrow` wrapper or suppress the affected IDE inspection, but neither workaround provides the correct library metadata.

Locate the NUnit.Framework external-annotation XML and compare the existing Assert.DoesNotThrow(TestDelegate) entries with the two System.Action overloads described in the issue. Add matching InstantHandle annotations for both code parameters, then validate the supplied NUnit/Rider reproduction no longer reports the closure or disposal warning.

Written by the indexing model from the issue text.