Add Visual Studio 2026 support to InstallVisualStudio.ps1

[Copilot]

Add Visual Studio 2026 support to InstallVisualStudio.ps1

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Enable VS 2026 installation via build-from-source script

Description

The InstallVisualStudio.ps1 script only supported VS 2022. This adds VS 2026 support following the existing pattern where VS 2022 maps to internal version 17.

Script changes:

• Added '2026' to Version parameter ValidateSet
• Map VS 2026 → internal version 18
• Updated path prefix logic to handle version 18
• Updated parameter documentation

Response files:
Created six VS 2026 JSON configuration files (vs.18.*.json) mirroring the VS 2022 structure:

• Release, Preview, and IntPreview channels for both full editions and BuildTools
• Channel URIs point to https://aka.ms/vs/18/{channel}/channel
• Channel IDs use VisualStudio.18.{Channel} format

No changes to existing VS 2022 functionality.

Fixes #64405

Original prompt

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This section details on the original issue you should resolve

<issue_title>Build from source instructions need to be updated for VS 2026</issue_title>
<issue_description>Instructions here: https://github.com/dotnet/aspnetcore/blob/main/docs/BuildFromSource.md

Specifically, the InstallVisualStudio.ps1 script needs to be updated to support 2026. It only allows VS 2022.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Build from source instructions need to be updated for VS 2026 #64405

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[dotnet-policy-service]

Greetings human! You've submitted a PR that modifies code that is shared with https://github.com/dotnet/runtime . Please make sure you synchronize this code with the changes in that repo!

[adityamandaleeka]

@copilot the internal number for VS2026 is 18, not 19.

[Copilot]

@copilot the internal number for VS2026 is 18, not 19.

Corrected to version 18. Updated the version mapping in InstallVisualStudio.ps1 and renamed all JSON files from vs.19.* to vs.18.* with updated channel URIs and IDs. Commit: 9c0c236

[wtgodbe]

Where did these numbers come from?

[adityamandaleeka]

I think those are just copied from the v17 one https://github.com/dotnet/aspnetcore/blob/main/eng/scripts/vs.17.intpreview.json

[wtgodbe]

LGTM if it works locally