Add warning when using --framework with solution files

[Copilot]

Using --framework on a solution build causes all projects to build for that framework, leading to referenced projects building multiple times and potential file lock contention.

Changes

• Added NETSDK1234 warning: New warning resource in Strings.resx that fires when --framework is used on solution builds
• Modified FrameworkOption: Updated CommonOptions.cs to forward _CommandLineDefinedTargetFramework=true property when --framework CLI option is used
• Added MSBuild target: Created _CheckForSolutionLevelTargetFramework in Microsoft.NET.Sdk.Solution.targets to detect and warn before Build/Publish/Clean/Test targets

Behavior

# Shows warning
$ dotnet build MySolution.sln --framework net8.0
warning NETSDK1234: The "--framework" option isn't supported when building a solution...

# No warning - property set directly
$ dotnet build MySolution.sln --property:TargetFramework=net8.0

Follows the same pattern as the existing --output warning (NETSDK1194).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>dotnet build --framework builds referenced project twice</issue_title>
<issue_description><!--
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Describe the bug

dotnet build with --framework builds the same project twice in parallel, if it is referenced by a solution and a ProjectReference. Without --framework, it builds only once.

To Reproduce

Dep.sln

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio Version 16
VisualStudioVersion = 16.0.30114.105
MinimumVisualStudioVersion = 10.0.40219.1
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "One", "One\One.csproj", "{6C41A8DE-5C2D-4D36-9F5E-5070A5333DC9}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Two", "Two\Two.csproj", "{8DEDAFB4-3647-450C-BD9A-0D1978A0A950}"
EndProject
Global
        GlobalSection(SolutionConfigurationPlatforms) = preSolution
                Debug|Any CPU = Debug|Any CPU
                Release|Any CPU = Release|Any CPU
        EndGlobalSection
        GlobalSection(SolutionProperties) = preSolution
                HideSolutionNode = FALSE
        EndGlobalSection
        GlobalSection(ProjectConfigurationPlatforms) = postSolution
                {6C41A8DE-5C2D-4D36-9F5E-5070A5333DC9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
                {6C41A8DE-5C2D-4D36-9F5E-5070A5333DC9}.Debug|Any CPU.Build.0 = Debug|Any CPU
                {6C41A8DE-5C2D-4D36-9F5E-5070A5333DC9}.Release|Any CPU.ActiveCfg = Release|Any CPU
                {6C41A8DE-5C2D-4D36-9F5E-5070A5333DC9}.Release|Any CPU.Build.0 = Release|Any CPU
                {8DEDAFB4-3647-450C-BD9A-0D1978A0A950}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
                {8DEDAFB4-3647-450C-BD9A-0D1978A0A950}.Debug|Any CPU.Build.0 = Debug|Any CPU
                {8DEDAFB4-3647-450C-BD9A-0D1978A0A950}.Release|Any CPU.ActiveCfg = Release|Any CPU
                {8DEDAFB4-3647-450C-BD9A-0D1978A0A950}.Release|Any CPU.Build.0 = Release|Any CPU
        EndGlobalSection
EndGlobal

One/One.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.XmlSerializer.Generator" Version="6.0.0" />
  </ItemGroup>

  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.XmlSerializer.Generator" Version="6.0.0" />
  </ItemGroup>

</Project>

One/Class1.cs

namespace One
{
    [System.Xml.Serialization.XmlRoot]
    public class Class1
    {
    }
}

Two/Two.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <ProjectReference Include="..\One\One.csproj" />
  </ItemGroup>

</Project>

Log of dotnet build --framework net6.0 becoming unexpectedly parallel and attempting to access a locked file

$ dotnet build --framework net6.0
Microsoft (R) Build Engine version 17.2.0+41abc5629 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  All projects are up-to-date for restore.
  One -> C:\Projects\Dep\One\bin\Debug\net6.0\One.dll
  One -> C:\Projects\Dep\One\bin\Debug\net6.0\One.dll
  .NET Xml Serialization Generation Utility, Version 6.0.0]
  Serialization Code File Name: C:\Projects\Dep\One\obj\Debug\net6.0\One.XmlSerializers.cs.
  Generated serialization code for assembly C:\Projects\Dep\One\obj\Debug\net6.0\One.dll --> 'C:\Projects\Dep\One\obj\Debug\net6.0\One.X
  mlSerializers.cs'.
  .NET Xml Serialization Generation Utility, Version 6.0.0]
  Serialization Code File Name: C:\Projects\Dep\One\obj\Debug\net6.0\One.XmlSerializers.cs.
  Generated serialization code for assembly C:\Projects\Dep\One\obj\Debug\net6.0\One.dll --> 'C:\Projects\Dep\One\obj\Debug\net6.0\One.X
  mlSerializers.cs'.
CSC : warning CS1504: Source file 'C:\Projects\Dep\One\obj\Debug\net6.0\One.XmlSerializers.cs' could not be opened -- The process cannot
 access the file 'C:\Projects\Dep\One\obj\Debug\net6.0\One.XmlSerializers.cs' because it is being used by another ...

</details>

- Fixes dotnet/sdk#26690

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[marcpopMSFT]

@baronfel in the spirit of limiting new warnings, should we only spit out this warning if the TFM is net11? People are definitely going to hit it.

[baronfel]

The problem (similar to the output path) is that we don't know at the solution-level that these targets run at if the projects target 11.

We probably should create a Target similar to what Restore does and have the solution gather the TFMs of the project and flag if the TFM is present.

[KalleOlaviNiemitalo]

Does this warn when dotnet test --no-build is used with --framework on a solution? I expect it shouldn't.