Add sdk.paths and sdk.errorMessage to global.json

[Copilot]

This change mirrors aspnetcore PR 62619 by adding two new properties to the sdk section of global.json:

• paths: [ ".dotnet", "$host$" ] - Instructs the .NET CLI to probe the repository-local .dotnet directory before falling back to host locations, enabling automatic use of the pinned local SDK without requiring manual PATH modifications.

• errorMessage: Provides clear guidance to developers when the SDK is missing, directing them to run the appropriate restore script (./restore.sh for Linux/macOS or ./restore.cmd for Windows).

Benefits

1. Improved Developer Experience: Developers no longer need to manually configure their PATH or DOTNET_ROOT to use the pinned SDK version.
2. Automatic SDK Resolution: The .NET CLI will automatically find and use the local SDK installation in the .dotnet directory.
3. Better Error Messaging: When the local SDK is missing, developers get actionable guidance instead of generic error messages.
4. Consistency: Aligns with the same approach being used in the aspnetcore repository.

Changes

The modification is minimal and surgical - only two properties were added to the existing sdk section:

 "sdk": {
   "version": "10.0.100-preview.5.25277.114",
   "rollForward": "major",
-  "allowPrerelease": true
+  "allowPrerelease": true,
+  "paths": [ ".dotnet", "$host$" ],
+  "errorMessage": "The .NET SDK could not be found. Run ./restore.sh (Linux/macOS) or ./restore.cmd (Windows) to install the local SDK."
 },

All other sections (tools, runtimes, msbuild-sdks) remain completely unchanged, and the existing SDK version and configuration are preserved.

Verification

• ✅ Build system continues to work with the pinned SDK version
• ✅ Local .dotnet directory is correctly prioritized for SDK resolution
• ✅ Fallback to host SDK works when local SDK is unavailable
• ✅ JSON syntax is valid
• ✅ No impact on existing tools, runtimes, or MSBuild SDK configurations

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Pull Request Overview

This PR adds two new properties to the sdk section of global.json to improve developer experience and SDK resolution. The change enables automatic discovery of the repository-local .NET SDK installation and provides clear error messaging when the SDK is missing.

• Adds paths property to prioritize local .dotnet directory for SDK resolution
• Adds errorMessage property to guide developers when the SDK is not found
• Maintains all existing SDK configuration and version settings

[davidfowl]

cc @baronfel @JakeRadMSFT

[mitchdenny]

Looks good. Did an extra check on codespaces.

[baronfel]

I was gonna suggest pumping the brakes a little bit on this - there's a open tooling issue that we're trying to work on at the IDE/tooling layers when local SDKs are used. But that problem mostly arises when building apps that target a runtime that isn't available outside of the 'local' SDK root. Since Aspire is still targeting the 8 TFM you should mostly sidestep the issue.