macOS: Update platform node when AXNodeData role changes

[knopp]

Fixes flutter/flutter#151428

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[cbracken]

[cbracken]

I kind of wonder if we should jam a debug assertion in shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMac.mm in the code near line 165 where we're casting directly to FlutterTextField unchecked. Something similar-ish to:

--- i/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMac.mm
+++ w/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMac.mm
@@ -165,11 +165,12 @@
         // If it is a text field, the value change notifications are handled by
         // the FlutterTextField directly. Only need to make sure it is the
         // first responder.
-        FlutterTextField* native_text_field =
-            (FlutterTextField*)mac_platform_node_delegate->GetNativeViewAccessible();
+        id native_text_field = mac_platform_node_delegate->GetNativeViewAccessible();
+        FML_DCHECK([native_text_field isKindOfClass:FlutterTextField.class]);
         id focused = mac_platform_node_delegate->GetFocus();
         if (!focused || native_text_field == focused) {
-          [native_text_field startEditing];
+          [(FlutterTextField*)native_text_field startEditing];
         }
         break;
       }

Cast at the bottom is technically unnecessary, but nice to catch if we ever renamed startEditing.