Solution-wide package restore fails

### Describe the bug

Updating nuget packages for a Visual Studio Solution may fail if multiple projects depend on the same NuGet pacakge.

It all seems to be caused by this file:\
[src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.CSharp.props](https://github.com/dotnet/sdk/blob/main/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.CSharp.props)

Projects that don't have a `` declaration will inherit `$(WarningsAsErrors);NU1605` from this file at line 18.\
This causes nuget upgrade to fail after the first project, since projects may have warning `NU1605: Detected package downgrade` while the upgrade goes through each project.

Since this warning is treated as an error, the package upgrade stops after the first project has had it's NuGet packages updated.

### To Reproduce

Unsure of how many cross-dependent projects need to exist for this to occur, but my solution contains 10 projects (half of them tests), that depend on one another.
```
Library1.csproj (=> Microsoft.Extensions.DependencyInjection.Abstractions 9.0.9)
Project1.csproj (=> Microsoft.Extensions.DependencyInjection.Abstractions 9.0.9, Library1)
```

Using the Visual Studio `Manage NuGet packages for Solution` menu for upgrading `Microsoft.Extensions.DependencyInjection.Abstractions` across both projects fails after `Library1` has been updated.

### Exceptions (if any)
```

error NU1605: Warning As Error: Detected package downgrade: Microsoft.Extensions.DependencyInjection.Abstractions from 9.0.11 to 9.0.9. Reference the package directly from the project to select a different version.
Project1 -> Library1 ->Microsoft.Extensions.DependencyInjection.Abstractions (>= 9.0.11)
Project1 -> Microsoft.Extensions.DependencyInjection.Abstractions (>= 9.0.9)
```

### Further technical details
details of dotnet --info

.NET SDK:
Version: 10.0.101
Commit: fad253f51b
Workload version: 10.0.100-manifests.9f71effe
MSBuild version: 18.0.6+fad253f51

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.101\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.1
Architecture: x64
Commit: fad253f51b

.NET SDKs installed:
10.0.101 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
DOTNET_CLI_UI_LANGUAGE [en-US]
DOTNET_GCHeapCount [2]
DOTNET_GCNoAffinitize [1]
DOTNET_MULTILEVEL_LOOKUP [0]
DOTNET_TC_CallCountThreshold [1000]
DOTNET_ThreadPool_UnfairSemaphoreSpinLimit [0]
DOTNET_gcConcurrent [0]
Detected COMPlus_* environment variable(s). Consider transitioning to DOTNET_* equivalent.

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

- Visual Studio 2026 18.1.0

Start by inspecting src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.CSharp.props, especially the WarningsAsErrors setting at line 18. Reproduce the failure with two dependent projects using Visual Studio's Manage NuGet Packages for Solution flow and the package versions shown. Done means solution-wide package restore or upgrade completes without stopping on NU1605.

Written by the indexing model from the issue text.