Announce changes to live regions on macOS (#21045)

* Announce changes to live regions on macOS

It is necessary to manually request announcement of the new name of a live
region, alongside posting the general kAXLiveRegionChangedNotification.

* Refactor -raisePropertyChanged: to use switch statement

* Fix warning about switch not covering all cases

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

native/Avalonia.Native/src/OSX/automation.mm

@@ -130,7 +130,7 @@
         case AutomationExpander: return NSAccessibilityDisclosureTriangleRole;
         // Treat unknown roles as generic group container items. Returning
         // NSAccessibilityUnknownRole is also possible but makes the screen
-       // reader focus on the item instead of passing focus to child items.
+        // reader focus on the item instead of passing focus to child items.
         default: return NSAccessibilityGroupRole;
     }
 }
@@ -191,6 +191,7 @@
     {
         switch (_peer->GetLiveSetting())
         {
+            case LiveSettingOff: return nil;
             case LiveSettingPolite: return @"polite";
             case LiveSettingAssertive: return @"assertive";
         }
@@ -519,6 +520,35 @@
 - (void)raiseLiveRegionChanged
 {
     NSAccessibilityPostNotification(self, @"AXLiveRegionChanged" /* kAXLiveRegionChangedNotification */);
+
+    // Announce the new string
+    auto name = _peer->GetName();
+    if (name != nullptr)
+    {
+        NSAccessibilityPriorityLevel priority = NSAccessibilityPriorityLow;
+        switch (_peer->GetLiveSetting())
+        {
+            case LiveSettingOff:
+                return;
+            case LiveSettingPolite:
+                priority = NSAccessibilityPriorityMedium;
+                break;
+            case LiveSettingAssertive:
+                priority = NSAccessibilityPriorityHigh;
+                break;
+        }
+
+        NSDictionary <NSString *, id> *userInfo = @{
+            NSAccessibilityAnnouncementKey: GetNSStringAndRelease(name),
+            NSAccessibilityPriorityKey: @(priority)
+        };
+
+        id topLevel = [self accessibilityTopLevelUIElement];
+        if ([topLevel isKindOfClass:[AvnWindow class]])
+        {
+            NSAccessibilityPostNotificationWithUserInfo(topLevel, NSAccessibilityAnnouncementRequestedNotification, userInfo);
+        }
+    }
 }
 
 - (void)setAccessibilityFocused:(BOOL)accessibilityFocused