Support major.minor version format in global.json runtimes

[Copilot]

Enables specifying major.minor versions (e.g., "8.0", "10.0") in global.json runtimes, which automatically installs the latest patch for that channel.

Changes

• InstallDotNetCore.cs: Detect two-part versions and use -channel parameter instead of -version
• eng/common scripts: Added -channel parameter support to dotnet-install.sh/ps1 and tools.sh/ps1
• Tests: Added 12 unit tests validating version format detection

Usage

{
  "tools": {
    "runtimes": {
      "aspnetcore": ["8.0", "10.0"]
    }
  }
}

Three-part versions continue to work unchanged.

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature request: Support major.minor in global.json for aspnetcore</issue_title>
<issue_description>

• This issue is blocking
• This issue is causing unreasonable pain

There is currently no mechanism such as darc/maestro to automatically keep up to date the version of aspnetcore that is referenced in a global.json. The dotnet/wcf repo has a global.json file that looks similar to the following:

{
  "tools": {
    "dotnet": "10.0.100",
    "runtimes": {
      "aspnetcore": [
        "8.0.22",
        "10.0.1"
      ]
    }
  },
  "sdk": {
    "version": "10.0.100"
  },
  "msbuild-sdks": {
    "Microsoft.DotNet.Arcade.Sdk": "11.0.0-beta.25615.1",
    "Microsoft.DotNet.Helix.Sdk": "11.0.0-beta.25615.1"
  }
}

It's a manual process to keep updating the aspnetcore version as new releases come out, and as a result often gets missed, sometimes for long durations of time. It would be sufficient for WCF's needs to simply support specifying major.minor for the versions and automatically using the latest version that's available.</issue_description>

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

@akoeplinger This will need some changes in https://github.com/dotnet/arcade/blob/47a8a69721dfea57b82121ac1458d2f5bba6abd2/src/Microsoft.DotNet.Arcade.Sdk/src/InstallDotNetCore.cs#L78-L128

We need to detect a two-part version like "8.0" or "10.0" and then pass that to the -channel parameter of the dotnet-install script: https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-install-script#options

-Channel|--channel <CHANNEL>

Specifies the source channel for the installation. The possible values are:

STS: The most recent Standard Term Support release.
LTS: The most recent Long Term Support release.
Two-part version in A.B format, representing a specific release (for example, 3.1 or 8.0).
Three-part version in A.B.Cxx format, representing a specific SDK release (for example, 8.0.1xx or 8.0.2xx). Available since the 5.0 release.
The version parameter overrides the channel parameter when any version other than latest is used.
```</body></comment_new>
</comments>

• Fixes Feature request: Support major.minor in global.json for aspnetcore #16419

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[akoeplinger]

@mconnew would this be a problem for you? it means we'd run dotnet-install.ps1 everytime