Microsoft.EntityFrameworkCore.Specification.Tests 11.0 RC1 cannot be restored from NuGet.org because of non-public Microsoft.DotNet.XUnitV3Extensions dependency

### Environment

- EF Core: 11.0.0-rc.1.26425.128
- NuGet source: https://api.nuget.org/v3/index.json
- Third-party EF Core provider consuming the shared specification test package

### Problem

`Microsoft.EntityFrameworkCore.Specification.Tests` at the current EF Core 11 RC version cannot be restored from public NuGet.org sources because it depends on `Microsoft.DotNet.XUnitV3Extensions`, which is not published to NuGet.org at all (404 on package index) at any version.

A third-party provider attempting to align its EF11 test dependencies with RC1 therefore cannot consume the matching `Specification.Tests` package.

### Reproduction

Create a clean test project and reference:

```xml

```

Restore using only NuGet.org:

```bash
dotnet restore
```

### Actual behavior

```text
error NU1101: Unable to find package Microsoft.DotNet.XUnitV3Extensions. No packages exist with this id in source(s): nuget.org
```

The `Microsoft.EntityFrameworkCore.Specification.Tests` 11.0.0-rc.1.26425.128 nuspec declares:

```xml

```

`Microsoft.DotNet.XUnitV3Extensions` returns HTTP 404 from `https://api.nuget.org/v3-flatcontainer/microsoft.dotnet.xunitv3extensions/index.json` — it is not published publicly at any version.

### Expected behavior

A publicly published `Microsoft.EntityFrameworkCore.Specification.Tests` package intended for database-provider testing should be restorable by third-party provider authors from the public package sources required by that package.

### Workaround

The current workaround is to pin:

```text
Microsoft.EntityFrameworkCore.Specification.Tests 11.0.0-preview.5.26302.115
```

while the remaining EF Core 11 runtime/design/tooling dependencies use:

```text
11.0.0-rc.1.26425.128
```

That restores successfully — confirmed via clean restore — but it forces provider tests to run against an older EF Core specification-test assembly than the actual EF version being targeted.

### First affected version

`11.0.0-preview.5.26302.115` restores cleanly (no `Microsoft.DotNet.XUnitV3Extensions` dependency in its nuspec). The dependency first appears in `11.0.0-preview.6.26359.118` and is still present, unchanged, in `11.0.0-preview.7.26381.103` and `11.0.0-rc.1.26425.128`.

### Impact

Third-party EF Core provider authors cannot validate against the matching EF11 RC shared specification suite.

This creates version skew exactly where provider compatibility testing is most important during the EF Core release-candidate cycle.

### Note

This is filed as a consumer-side restore problem, not a request to publish Microsoft's internal test infrastructure specifically — the fix could equally be removing/replacing that dependency in the public package, preventing it from flowing transitively, or publishing it. Leaving the choice to the maintainers.