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/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