Clarify Darc minimum-version upgrade instruction based on host OS

[Copilot]

The Darc minimum-version error was pointing users to a generic darc-init command, which obscured the actual script to run on each platform. This updates the guidance to select the correct bootstrap script for the current host OS at runtime.

• What changed

  • Updated DarcVersionValidator's minimum-version error message to reference a single OS-appropriate script:
    • ./eng/common/darc-init.sh on Linux/macOS
    • .\eng\common\darc-init.ps1 on Windows
  • Kept the existing dotnet tool update -g microsoft.dotnet.darc fallback in the message.

• Why it matters

  • Makes upgrade instructions immediately actionable without showing users commands for other platforms.
  • Aligns the Darc CLI error text with the host machine OS instead of requiring users to choose between multiple script names.

• Resulting message

  logger.LogError(
      "Your darc version {DarcVersion} is below the minimum required version {MinVersion}. Run `{DarcInitCommand}` (or `dotnet tool update -g microsoft.dotnet.darc`) to upgrade.",
      darcVersion.ToNormalizedString(),
      minVersion.ToNormalizedString(),
      GetDarcInitCommand());

[premun]

@copilot can you just show the right one based on the host machine OS info?

[Copilot]

Updated in f7660e7 — the validator now selects a single command at runtime based on the host OS: ./eng/common/darc-init.sh on Linux/macOS and .\eng\common\darc-init.ps1 on Windows.

[Copilot]

Pull request overview

Updates the Darc CLI minimum-version enforcement message so users are guided to run the correct bootstrap script for their current OS, making the upgrade path more immediately actionable.

Changes:

• Updated the minimum-version failure log message to reference an OS-appropriate eng/common/darc-init script.
• Added a small helper (GetDarcInitCommand) that selects the script path based on the host OS at runtime.