Extract release tag derivation into a testable helper

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

.github/actions/Publish-PSModule/src/Publish-PSModule.Helpers.psm1

@@ -0,0 +1,44 @@
+function Get-ReleaseTag {
+    <#
+        .SYNOPSIS
+        Builds the git tag used for the GitHub release.
+
+        .DESCRIPTION
+        Composes the release tag from the module version and, when present, the prerelease label.
+        The version comes from the compiled manifest, which is the artifact that is published, so the
+        tag always names the exact bytes that were tested and pushed to the PowerShell Gallery.
+
+        .OUTPUTS
+        String with the release tag.
+
+        .EXAMPLE
+        Get-ReleaseTag -ModuleVersion '1.1.10'
+
+        Returns '1.1.10'.
+
+        .EXAMPLE
+        Get-ReleaseTag -ModuleVersion '1.1.10' -Prerelease 'mybranch001'
+
+        Returns '1.1.10-mybranch001'.
+    #>
+    [CmdletBinding()]
+    [OutputType([string])]
+    param(
+        # The module version from the compiled manifest, in Major.Minor.Patch format.
+        [Parameter(Mandatory)]
+        [ValidateNotNullOrEmpty()]
+        [string] $ModuleVersion,
+
+        # The prerelease label from the compiled manifest. Empty for a stable release.
+        [Parameter()]
+        [AllowEmptyString()]
+        [AllowNull()]
+        [string] $Prerelease
+    )
+
+    if ([string]::IsNullOrWhiteSpace($Prerelease)) {
+        return $ModuleVersion
+    }
+
+    "$ModuleVersion-$($Prerelease.Trim())"
+}

.github/actions/Publish-PSModule/src/publish.ps1

@@ -28,6 +28,7 @@ param()
 $PSStyle.OutputRendering = 'Ansi'
 
 Import-Module -Name 'PSModule' -Force
+Import-Module -Name "$PSScriptRoot/Publish-PSModule.Helpers.psm1" -Force
 
 #region Load inputs
 LogGroup 'Load inputs' {
@@ -129,7 +130,7 @@ LogGroup 'Resolve version from manifest' {
         $createPrerelease = $true
     }
 
-    $releaseTag = if ($createPrerelease) { "$moduleVersion-$prerelease" } else { $moduleVersion }
+    $releaseTag = Get-ReleaseTag -ModuleVersion $moduleVersion -Prerelease $prerelease
 
     [PSCustomObject]@{
         ModuleVersion    = $moduleVersion