Fix read only TextField focus traversal on macOS (#166056)

## Description

This PR fixes a focus traversal issue for read-only TextField on macOS.

# Implementation details

On macOS, some editing capabilities are handled differently compared to
other platforms.
Since https://github.com/flutter/flutter/pull/105407, the macOS engine
send editing selectors to the framework.
To do so a text input connection should be opened.

Before this PR there was no text input connection for a read-only
EditableText which means several shortcut were not handled, especially
tab traversal (but also selection shortcuts using arrows).

After this PR an input connection is always created on macOS even if an
EditableText is read-only.

## Related Issue

Fixes [Read-only TextField prevents focus from
changing](https://github.com/flutter/flutter/issues/161482)

## Tests

Adds 1 test.

packages/flutter/lib/src/widgets/editable_text.dart

@@ -2469,14 +2469,19 @@ class EditableTextState extends State<EditableText>
   /// Read-only input fields do not need a connection with the platform since
   /// there's no need for text editing capabilities (e.g. virtual keyboard).
   ///
+  /// On macOS, most of the selection and focus related shortcuts require a
+  /// connection with the platform because appropriate platform selectors are
+  /// sent from the engine and translated into intents. For read-only fields
+  /// those shortcuts should be available (for instance to allow tab traversal).
+  ///
   /// On the web, we always need a connection because we want some browser
   /// functionalities to continue to work on read-only input fields like:
-  ///
   /// - Relevant context menu.
   /// - cmd/ctrl+c shortcut to copy.
   /// - cmd/ctrl+a to select all.
   /// - Changing the selection using a physical keyboard.
-  bool get _shouldCreateInputConnection => kIsWeb || !widget.readOnly;
+  bool get _shouldCreateInputConnection =>
+      kIsWeb || defaultTargetPlatform == TargetPlatform.macOS || !widget.readOnly;
 
   // The time it takes for the floating cursor to snap to the text aligned
   // cursor position after the user has finished placing it.

packages/flutter/test/widgets/editable_text_test.dart

@@ -3013,6 +3013,62 @@ void main() {
     }
   });
 
+  testWidgets(
+    'Read-only fields can be traversed on all platforms',
+    (WidgetTester tester) async {
+      final TextEditingController controller1 = TextEditingController();
+      addTearDown(controller1.dispose);
+      final TextEditingController controller2 = TextEditingController();
+      addTearDown(controller2.dispose);
+      final FocusNode focusNode1 = FocusNode();
+      addTearDown(focusNode1.dispose);
+      final FocusNode focusNode2 = FocusNode();
+      addTearDown(focusNode2.dispose);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Column(
+            children: <Widget>[
+              EditableText(
+                focusNode: focusNode1,
+                autofocus: true,
+                controller: controller1,
+                backgroundCursorColor: Colors.grey,
+                style: textStyle,
+                cursorColor: cursorColor,
+              ),
+              EditableText(
+                readOnly: true,
+                focusNode: focusNode2,
+                controller: controller2,
+                backgroundCursorColor: Colors.grey,
+                style: textStyle,
+                cursorColor: cursorColor,
+              ),
+            ],
+          ),
+        ),
+      );
+
+      expect(focusNode1.hasPrimaryFocus, true);
+      expect(focusNode2.hasPrimaryFocus, false);
+
+      // Change focus to the readonly EditableText.
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+
+      expect(focusNode1.hasPrimaryFocus, false);
+      expect(focusNode2.hasPrimaryFocus, true);
+
+      // Change focus back to the first EditableText.
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+
+      expect(focusNode1.hasPrimaryFocus, true);
+      expect(focusNode2.hasPrimaryFocus, false);
+    },
+    variant: TargetPlatformVariant.all(),
+    skip: kIsWeb, // [intended]
+  );
+
   testWidgets('Sends "updateConfig" when read-only flag is flipped', (WidgetTester tester) async {
     bool readOnly = true;
     late StateSetter setState;