Diagnostic suppressor seems to work only for open files

Hi,

this is somewhat of a follow-up to https://github.com/dotnet/roslyn/issues/68885. I've initially reported this to https://github.com/nunit/nunit.analyzers/issues/786 as well, but it doesn't seem to be an issue with the analyzer itself.

For context: The NUnit.Analyzer package provides diagnostic suppressor to suppress nullable warnings for objects that can be null, but have previously been checked with NUnit's asserts that they are not null.

We recently changed a legacy test project that previously had NRTs disabled to enable them. We saw hundreds of "CS8602 - Dereference of a possibly null reference" warnings for objects that can be in fact null, but were checked with some variation of "Assert this is not null". Curiously, the warnings weren't reported at build time and disappeared after the file with the warning was opened.

Here's a repro, mostly based on the Roslyn issue above:
Project file:
```xml

net8.0
false
false
true
enable
enable

true
true

NU1608


all
runtime; build; native; contentfiles; analyzers; buildtransitive

```

Test:
```csharp
using NUnit.Framework;

namespace NUnitNullabilityTest
{
[TestFixture]
public class Tests
{
private class Model
{
public string? Name { get; set; }

public string? SomeMethod() => Name;
}

[Test]
public void Model_SomeMethod()
{
var target = new Model { Name = "name" };

var result = target.SomeMethod();

Assert.That(result, Is.Not.Null);
Assert.That(result.Length, Is.EqualTo(4)); // This is of course a nonsense assertion, but repros the issue
}
}
}

```

`result` can technically be null, so `result.Length` could theoretically throw an NRE. Since `Is.Not.Null` is asserted, we can be sure it's not null.

This produces the warning if the file is not opened:

An vanishes if the file is opened:

Start with the provided project file and the Tests.Model_SomeMethod repro, comparing build diagnostics before and after opening the file. Then investigate Roslyn's diagnostic-suppressor handling for closed documents. Done means the NUnit nullable suppression is applied consistently at build time and opening the file no longer changes CS8602.

Written by the indexing model from the issue text.