ci: consolidate platform pipelines and test prerequisites on macOS support

MeetingAssistant.Tests/WindowsFactAttribute.cs

@@ -0,0 +1,12 @@
+namespace MeetingAssistant.Tests;
+
+public sealed class WindowsFactAttribute : FactAttribute
+{
+    public WindowsFactAttribute(string requiredCapability)
+    {
+        if (!OperatingSystem.IsWindows())
+        {
+            Skip = $"Requires Windows {requiredCapability}; run on Windows.";
+        }
+    }
+}