fix(build): add netstandard2.0 target to eliminate System.Runtime version mismatch

The net9.0 DLL references System.Runtime 9.0.0.0 which is unavailable
on GitHub runners where pwsh ships with .NET 8.x. Adding netstandard2.0
as a target produces a DLL compatible with any .NET version (PS 5.1 on
.NET Framework through PS 7.x on .NET 8/9).

- Add netstandard2.0 to TargetFrameworks in Core and module csproj
- Update #if NET48 conditionals to include NETSTANDARD2_0
- Switch CI workflows from net9.0 to netstandard2.0 for Pester tests
- Update _TestHelper.ps1 framework search order

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/integration-tests.yml

@@ -178,10 +178,10 @@ jobs:
       # ── Build and test ─────────────────────────────────────────────────
 
       - name: Build module
-        run: dotnet publish src/PSProxmoxVE/PSProxmoxVE.csproj --configuration Release --framework net9.0 --output ./publish/net9.0
+        run: dotnet publish src/PSProxmoxVE/PSProxmoxVE.csproj --configuration Release --framework netstandard2.0 --output ./publish/netstandard2.0
 
       - name: Clean publish output for PS module loading
-        run: rm -f ./publish/net9.0/*.deps.json ./publish/net9.0/*.runtimeconfig.json
+        run: rm -f ./publish/netstandard2.0/*.deps.json
 
       - name: Install Pester 5
         shell: pwsh
@@ -192,7 +192,7 @@ jobs:
         run: |
           $modulePath = "$HOME/.local/share/powershell/Modules/PSProxmoxVE"
           New-Item -ItemType Directory -Path $modulePath -Force | Out-Null
-          Copy-Item -Path ./publish/net9.0/* -Destination $modulePath -Recurse -Force
+          Copy-Item -Path ./publish/netstandard2.0/* -Destination $modulePath -Recurse -Force
 
       - name: Create test ISO
         shell: bash
@@ -201,8 +201,6 @@ jobs:
       - name: Run integration tests
         shell: pwsh
         env:
-          DOTNET_ROOT: ''
-          DOTNET_MULTILEVEL_LOOKUP: ''
           PVETEST_HOST: ${{ steps.target.outputs.host }}
           PVETEST_PORT: ${{ steps.target.outputs.port }}
           PVETEST_APITOKEN: ${{ steps.target.outputs.token }}

.github/workflows/unit-tests.yml

@@ -18,9 +18,9 @@ jobs:
           - os: windows-latest
             framework: net48
           - os: windows-latest
-            framework: net9.0
+            framework: netstandard2.0
           - os: ubuntu-latest
-            framework: net9.0
+            framework: netstandard2.0
 
     steps:
       - uses: actions/checkout@v5
@@ -58,17 +58,17 @@ jobs:
             shell: powershell
           - os: windows-latest
             ps_version: '7.5'
-            framework: net9.0
+            framework: netstandard2.0
             artifact_os: windows-latest
             shell: pwsh
           - os: ubuntu-latest
             ps_version: '7.5'
-            framework: net9.0
+            framework: netstandard2.0
             artifact_os: ubuntu-latest
             shell: pwsh
           - os: macos-latest
             ps_version: '7.5'
-            framework: net9.0
+            framework: netstandard2.0
             artifact_os: ubuntu-latest
             shell: pwsh