Add SolutionX item template for creating an empty slnx file

[Copilot]

Adds a dedicated item template that creates an empty .slnx solution file. Output matches dotnet new sln (which already defaults to slnx), but this template intentionally exposes no --format option.

$ dotnet new slnx -n MySolution
The template "XML Solution File" was created successfully.

$ cat MySolution.slnx
<Solution>
</Solution>

Template

• New template_feed/Microsoft.DotNet.Common.ItemTemplates/content/SolutionX/ modeled on the existing Solution template:
  • Solution1.slnx content file only — no .sln variant, no Format symbol, no dotnetcli.host.json.
  • Identity Microsoft.Standard.QuickStarts.SolutionX, group ItemSolutionX, short names slnx / solutionx, name "XML Solution File".
• English templatestrings.en.json; other locales flow through the loc pipeline.

Tests

• AllCommonItemsCreate rows for both short names, with a new approval snapshot.
• slnx added to the CanDoTabCompletion approvals (new template group contributes its primary short name).

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[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
2 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

This PR adds a new common item template that generates an empty .slnx (XML solution) file, providing a direct dotnet new slnx/dotnet new solutionx experience without exposing a --format option (unlike the existing sln template).

Changes:

• Added a new item template under template_feed/.../SolutionX/ that emits an empty Solution1.slnx.
• Extended dotnet new integration coverage to create the template via both short names (slnx, solutionx) with an approval snapshot.
• Updated tab-completion approval baselines to include slnx across Windows/Linux/macOS.

Show a summary per file

File	Description
test/dotnet-new.IntegrationTests/CommonTemplatesTests.cs	Adds AllCommonItemsCreate coverage for the new XML Solution File template via slnx and solutionx.
test/dotnet-new.IntegrationTests/Approvals/DotnetNewCompleteTests.CanDoTabCompletion.Windows.verified.txt	Updates tab-completion baseline to include slnx.
test/dotnet-new.IntegrationTests/Approvals/DotnetNewCompleteTests.CanDoTabCompletion.OSX.verified.txt	Updates tab-completion baseline to include slnx.
test/dotnet-new.IntegrationTests/Approvals/DotnetNewCompleteTests.CanDoTabCompletion.Linux.verified.txt	Updates tab-completion baseline to include slnx.
test/dotnet-new.IntegrationTests/Approvals/AllCommonItemsCreate.-o#XML-Solution-File#-n#item.verified/XML-Solution-File/item.slnx	Adds approval snapshot for generated .slnx output.
test/dotnet-new.IntegrationTests/Approvals/AllCommonItemsCreate.-o#XML-Solution-File#-n#item.verified/std-streams/stdout.txt	Adds approval snapshot for command stdout for the new template scenario.
template_feed/Microsoft.DotNet.Common.ItemTemplates/content/SolutionX/Solution1.slnx	Adds the template content file for an empty .slnx.
template_feed/Microsoft.DotNet.Common.ItemTemplates/content/SolutionX/.template.config/template.json	Registers the new SolutionX item template (identity/group/short names/metadata).
template_feed/Microsoft.DotNet.Common.ItemTemplates/content/SolutionX/.template.config/localize/templatestrings.en.json	Adds English-localized strings for name/description/author.

Copilot's findings

• Files reviewed: 9/9 changed files
• Comments generated: 0