Backport VS Release Images fix to release/8.0.4xx

[Copilot]

Backports #51558 to release/8.0.4xx. VS Release images reject preview SDKs by default, causing MSBuild SDK resolver to fail finding internal SDKs during Helix test runs.

Changes

• New script eng/enable-preview-sdks.ps1: Modifies VS user settings to enable preview SDKs by setting UsePreviews=True in the VS instance-specific sdk.txt file

• Updated build/RunTestsOnHelix.cmd: Invokes enable-preview-sdks.ps1 before MSBuild path resolution to ensure preview SDKs are enabled during test execution

• Fixed eng/common/tools.ps1 LocateVisualStudio function: Safely handles missing vs property in global.json to prevent Arcade from failing when VS requirements are undefined

# Before: crashes if GlobalJson.tools.vs doesn't exist
if (!$vsRequirements) { $vsRequirements = $GlobalJson.tools.vs }

# After: gracefully handles missing property
if (!$vsRequirements) {
  if (Get-Member -InputObject $GlobalJson.tools -Name 'vs' -ErrorAction SilentlyContinue) {
    $vsRequirements = $GlobalJson.tools.vs
  }
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Backport VS Release Images fix</issue_title>
<issue_description>Please start a PR to backport https://github.com/dotnet/sdk/pull/51558/checks?check_run_id=54843914657 to 8.0.4xx. Also, if you can start multiple PRs, backport it to 8.0.1xx and 8.0.3xx as well.</issue_description>

<agent_instructions>Please backport the changes in https://github.com/dotnet/sdk/pull/51558/checks?check_run_id=54843914657 to the release/8.0.4xx branch.</agent_instructions>

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

• Fixes Backport VS Release Images fix #51625

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