Report read-only value state to macOS accessibility (#21597)

AvnAccessibilityElement answers accessibilityIsAttributeSettable for the value attribute using IValueProvider.IsReadOnly, so AX clients see a read-only TextBox as not settable.

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

@@ -274,6 +274,20 @@
     }
 }
 
+- (BOOL)accessibilityIsAttributeSettable:(NSAccessibilityAttributeName)attribute
+{
+    if ([attribute isEqualToString:NSAccessibilityValueAttribute])
+    {
+        if (_peer->IsValueProvider())
+            return !_peer->ValueProvider_IsReadOnly();
+        if (_peer->IsRangeValueProvider())
+            return YES;
+        return NO;
+    }
+
+    return [super accessibilityIsAttributeSettable:attribute];
+}
+
 - (id)accessibilityMinValue
 {
     if (_peer->IsRangeValueProvider())

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -201,6 +201,7 @@ namespace Avalonia.Native
         public int IsValueProvider() => IsProvider<IValueProvider>();
         public IAvnString? ValueProvider_GetValue() => ValueProvider.Value.ToAvnString();
         public void ValueProvider_SetValue(string value) => ValueProvider.SetValue(value);
+        public int ValueProvider_IsReadOnly() => ValueProvider.IsReadOnly.AsComBool();
 
         [return: NotNullIfNotNull("peer")]
         public static AvnAutomationPeer? Wrap(AutomationPeer? peer)

src/Avalonia.Native/avn.idl

@@ -1329,6 +1329,7 @@ interface IAvnAutomationPeer : IUnknown
      bool IsValueProvider();
      IAvnString* ValueProvider_GetValue();
      void ValueProvider_SetValue([const] char* value);
+     bool ValueProvider_IsReadOnly();
 
      IAvnString* GetHelpText();
      IAvnString* GetPlaceholderText();