test: cover Assert-VisualCRedistributableInstalled to maintain coverage after lazy probe

Adds InModuleScope tests for the private VC++ runtime probe so the deferred-init refactor (#54) does not regress code coverage.

Author: MariusStorhaug

tests/Sodium.Tests.ps1

@@ -166,4 +166,27 @@
                 Should -Throw 'Invalid private key. Expected 32 bytes but got 16.'
         }
     }
+
+    Context 'Runtime diagnostics' {
+        It 'Assert-VisualCRedistributableInstalled returns a boolean for the current architecture' {
+            InModuleScope Sodium {
+                $arch = if ([System.Environment]::Is64BitProcess) { 'X64' } else { 'X86' }
+                $result = Assert-VisualCRedistributableInstalled -Version '14.0' -Architecture $arch
+                $result | Should -BeOfType [bool]
+            }
+        }
+
+        It 'Assert-VisualCRedistributableInstalled treats non-Windows platforms as unsupported' {
+            InModuleScope Sodium {
+                $original = $IsWindows
+                try {
+                    Set-Variable -Name IsWindows -Scope Script -Value $false
+                    $result = Assert-VisualCRedistributableInstalled -Version '14.0' -Architecture 'X64' 3>$null
+                    $result | Should -BeFalse
+                } finally {
+                    Set-Variable -Name IsWindows -Scope Script -Value $original
+                }
+            }
+        }
+    }
 }