Set sdk.txt file to enable preview SDKs

- VS uses the local appdata, VS versioned folder, sdk.txt file to determine whether or not to accept preview sdks

- VS Release rejects preview SDKs by default which causes the msbuild sdk resolver to not find sdks, as msbuild respects the vs settings

- This changes the vs settings to enable our internal SDKs to be found by msbuild when it's going under test.

- Possible improvements: this PR assumes sdk.txt content ordering does not matter - based on VS code this appears to be true but could change in the future

Also tried:
- modifying global.json to enable preview sdks, which was not enough. some tests have a  custom global.json and it seems to not take precedence

setting env vars such as msbuildsdks which did not take precedence over the vs setting

This commit is a condensed version of the work done in #51591 and #51598

build/RunTestsOnHelix.cmd

@@ -10,6 +10,9 @@ set PATH=%DOTNET_ROOT%;%PATH%
 set DOTNET_MULTILEVEL_LOOKUP=0
 set TestFullMSBuild=%1
 
+REM Ensure Visual Studio instances allow preview SDKs
+PowerShell -ExecutionPolicy ByPass -NoProfile -File "%HELIX_CORRELATION_PAYLOAD%\t\eng\enable-preview-sdks.ps1"
+
 REM Use powershell to call partical Arcade logic to get full framework msbuild path and assign it
 if "%TestFullMSBuild%"=="true" (
     FOR /F "tokens=*" %%g IN ('PowerShell -ExecutionPolicy ByPass -File "%HELIX_CORRELATION_PAYLOAD%\t\eng\print-full-msbuild-path.ps1"') do (SET DOTNET_SDK_TEST_MSBUILD_PATH=%%g)

eng/enable-preview-sdks.ps1

@@ -0,0 +1,59 @@
+param()
+
+. $PSScriptRoot\common\tools.ps1
+
+try {
+    $vsInfo = LocateVisualStudio
+}
+catch {
+    Write-Host "LocateVisualStudio failed: $_"
+    return
+}
+
+if ($null -eq $vsInfo) {
+    Write-Host "No Visual Studio instance detected; preview SDKs remain enabled by default."
+    return
+}
+
+$vsId = $vsInfo.instanceId
+$vsMajorVersion = $vsInfo.installationVersion.Split('.')[0]
+$instanceDir = Join-Path $env:USERPROFILE "AppData\Local\Microsoft\VisualStudio\$vsMajorVersion.0_$vsId"
+
+Create-Directory $instanceDir
+
+$sdkFile = Join-Path $instanceDir 'sdk.txt'
+
+$desiredLine = 'UsePreviews=True'
+$existingLines = @()
+
+if (Test-Path $sdkFile) {
+    $existingLines = @(Get-Content -Path $sdkFile -Encoding ASCII)
+}
+
+# Determine how to place the UsePreviews flag based on existing content.
+$replacementIndex = -1
+for ($i = 0; $i -lt $existingLines.Count; $i++) {
+    if ($existingLines[$i] -match '^UsePreviews=.*$') {
+        $replacementIndex = $i
+        break
+    }
+}
+
+# Replace the existing line to enforce it as True
+if ($replacementIndex -ge 0) {
+    $updatedLines = $existingLines
+    $updatedLines[$replacementIndex] = $desiredLine
+}
+elseif ($existingLines.Count -gt 0) {
+    # Write to the top of the file but keep the remaining portion (assumption: order does not matter to VS)
+    $updatedLines = @($desiredLine) + $existingLines
+}
+else {
+    # Write a whole new file
+    $updatedLines = @($desiredLine)
+}
+
+Set-Content -Path $sdkFile -Value $updatedLines -Encoding ASCII
+
+Write-Host "Updated $sdkFile"
+Get-Content -Path $sdkFile | ForEach-Object { Write-Host "  $_" }