Fix failing "Update BC Artifact Version" job when BCPlatform is untracked (#9022)

The scheduled `[releases/26.4] Update BC Artifact Version` job fails
because the `UpdatePlatformVersion` automation calls
`Update-PackageVersion -PackageName "BCPlatform"`, which throws `Package
BCPlatform not found in Packages config`. Release branches like
`releases/26.4` don't track a `BCPlatform` entry in
`build/Packages.json`, so the automation errors and fails the whole job.

### Changes
- **`.github/actions/RunAutomation/UpdatePlatformVersion/run.ps1`**:
guard `Update-PackageVersion` with an existence check on the
`BCPlatform` package. When it's absent, log and return "No update
available" instead of throwing. Behavior is unchanged on branches that
track the package.

```powershell
$platformPackage = Get-ConfigValue -Key "BCPlatform" -ConfigType Packages
if (-not $platformPackage) {
    Write-Host "BCPlatform package is not present in the Packages config. Skipping update."
    return $result
}

$newVersion = Update-PackageVersion -PackageName "BCPlatform"
```



[AB#612711](https://dynamicssmb2.visualstudio.com/1fcb79e7-ab07-432a-a3c6-6cf5a88ba4a5/_workitems/edit/612711)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

.github/actions/RunAutomation/UpdatePlatformVersion/run.ps1

@@ -6,13 +6,21 @@ param (
 
 Import-Module $PSScriptRoot\..\..\..\..\build\scripts\EnlistmentHelperFunctions.psm1
 
-$newVersion = Update-PackageVersion -PackageName "BCPlatform"
-
 $result = @{
     'Files' = @()
     'Message' = "No update available"
 }
 
+# Defensively check whether the BCPlatform package is tracked in the Packages config.
+# Some (older) release branches do not track the BCPlatform package, in which case there is nothing to update.
+$platformPackage = Get-ConfigValue -Key "BCPlatform" -ConfigType Packages
+if (-not $platformPackage) {
+    Write-Host "BCPlatform package is not present in the Packages config. Skipping update."
+    return $result
+}
+
+$newVersion = Update-PackageVersion -PackageName "BCPlatform"
+
 if ($newVersion) {
     $result.Files = @(Get-PackagesFilePath -Relative)
     $result.Message = "Update platform version. New value: $newVersion"