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>

src/PSProxmoxVE.Core/Client/PveHttpClient.cs

@@ -10,7 +10,7 @@ using Newtonsoft.Json.Linq;
 using PSProxmoxVE.Core.Authentication;
 using PSProxmoxVE.Core.Exceptions;
 
-#if NET48
+#if NET48 || NETSTANDARD2_0
 using System.Net.Security;
 using System.Security.Cryptography.X509Certificates;
 #endif
@@ -35,7 +35,7 @@ namespace PSProxmoxVE.Core.Client
             _session = session ?? throw new ArgumentNullException(nameof(session));
             _baseUrl = session.BaseUrl;
 
-#if NET48
+#if NET48 || NETSTANDARD2_0
             var handler = new HttpClientHandler();
             if (session.SkipCertificateCheck)
             {
@@ -75,7 +75,7 @@ namespace PSProxmoxVE.Core.Client
             _session = null;
             _baseUrl = $"https://{hostname}:{port}";
 
-#if NET48
+#if NET48 || NETSTANDARD2_0
             var handler = new HttpClientHandler();
             if (skipCertificateCheck)
             {
@@ -286,7 +286,7 @@ namespace PSProxmoxVE.Core.Client
             }
             finally
             {
-#if NET48
+#if NET48 || NETSTANDARD2_0
                 fileStream.Dispose();
 #else
                 await fileStream.DisposeAsync().ConfigureAwait(false);

src/PSProxmoxVE.Core/PSProxmoxVE.Core.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net9.0;net48</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net9.0;net48</TargetFrameworks>
     <LangVersion>10.0</LangVersion>
     <Nullable>enable</Nullable>
     <RootNamespace>PSProxmoxVE.Core</RootNamespace>
@@ -16,6 +16,11 @@
     </AssemblyAttribute>
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+  </ItemGroup>
+
   <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="System.Text.Json" Version="8.0.5" />

src/PSProxmoxVE/PSProxmoxVE.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net9.0;net48</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net9.0;net48</TargetFrameworks>
     <LangVersion>10.0</LangVersion>
     <Nullable>enable</Nullable>
     <RootNamespace>PSProxmoxVE</RootNamespace>
@@ -14,7 +14,7 @@
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' Or '$(TargetFramework)' == 'net48'">
     <PackageReference Include="PowerShellStandard.Library" Version="5.1.1" PrivateAssets="all" />
   </ItemGroup>