CupertinoTextField accessibility behavior on Linux should match TextField (#159823)

While going through `TextField` and `CupertinoTextField` I noticed
https://github.com/flutter/flutter/pull/94898 and
https://github.com/flutter/flutter/pull/129652 did not make it over to
`CupertinoTextField`. This PR brings over those changes to
`CupertinoTextField`. On Linux after this change the
`CupertinoTextField` now focuses when gaining a11y focus, and unfocuses
when losing a11y focus.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Co-authored-by: Renzo Olivares <roliv@google.com>

packages/flutter/lib/src/cupertino/text_field.dart

@@ -1304,13 +1304,13 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
 
     TextSelectionControls? textSelectionControls = widget.selectionControls;
     VoidCallback? handleDidGainAccessibilityFocus;
+    VoidCallback? handleDidLoseAccessibilityFocus;
     switch (defaultTargetPlatform) {
       case TargetPlatform.iOS:
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
-      case TargetPlatform.linux:
         textSelectionControls ??= cupertinoTextSelectionHandleControls;
-
+      case TargetPlatform.linux:
       case TargetPlatform.macOS:
       case TargetPlatform.windows:
         textSelectionControls ??= cupertinoDesktopTextSelectionHandleControls;
@@ -1320,6 +1320,9 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
             _effectiveFocusNode.requestFocus();
           }
         };
+        handleDidLoseAccessibilityFocus = () {
+          _effectiveFocusNode.unfocus();
+        };
     }
 
     final bool enabled = widget.enabled;
@@ -1484,6 +1487,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
         _requestKeyboard();
       },
       onDidGainAccessibilityFocus: handleDidGainAccessibilityFocus,
+      onDidLoseAccessibilityFocus: handleDidLoseAccessibilityFocus,
       onFocus: enabled
         ? () {
             assert(

packages/flutter/test/cupertino/text_field_test.dart

@@ -503,7 +503,7 @@ void main() {
     );
   });
 
-  testWidgets('Activates the text field when receives semantics focus on Mac, Windows', (WidgetTester tester) async {
+  testWidgets('Activates the text field when receives semantics focus on desktops', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);
     final SemanticsOwner semanticsOwner = tester.binding.pipelineOwner.semanticsOwner!;
     final FocusNode focusNode = FocusNode();
@@ -538,6 +538,7 @@ void main() {
                           SemanticsAction.tap,
                           SemanticsAction.focus,
                           SemanticsAction.didGainAccessibilityFocus,
+                          SemanticsAction.didLoseAccessibilityFocus,
                         ],
                         textDirection: TextDirection.ltr,
                       ),
@@ -557,8 +558,11 @@ void main() {
     semanticsOwner.performAction(4, SemanticsAction.didGainAccessibilityFocus);
     await tester.pumpAndSettle();
     expect(focusNode.hasFocus, isTrue);
+    semanticsOwner.performAction(4, SemanticsAction.didLoseAccessibilityFocus);
+    await tester.pumpAndSettle();
+    expect(focusNode.hasFocus, isFalse);
     semantics.dispose();
-  }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.macOS, TargetPlatform.windows }));
+  }, variant: TargetPlatformVariant.desktop());
 
   testWidgets(
     'takes available space horizontally and takes intrinsic space vertically no-strut',
@@ -10458,8 +10462,13 @@ void main() {
                         actions: <SemanticsAction>[
                           SemanticsAction.tap,
                           SemanticsAction.focus,
-                          if (defaultTargetPlatform == TargetPlatform.windows || defaultTargetPlatform == TargetPlatform.macOS)
-                            SemanticsAction.didGainAccessibilityFocus,
+                          if (defaultTargetPlatform == TargetPlatform.linux
+                              || defaultTargetPlatform == TargetPlatform.windows
+                              || defaultTargetPlatform == TargetPlatform.macOS)
+                            ...<SemanticsAction>[
+                              SemanticsAction.didGainAccessibilityFocus,
+                              SemanticsAction.didLoseAccessibilityFocus,
+                            ],
                           // TODO(gspencergoog): also test for the presence of SemanticsAction.focus when
                           // this iOS issue is addressed: https://github.com/flutter/flutter/issues/150030
                         ],
@@ -10515,8 +10524,13 @@ void main() {
                           SemanticsFlag.isReadOnly,
                         ],
                         actions: <SemanticsAction>[
-                          if (defaultTargetPlatform == TargetPlatform.windows || defaultTargetPlatform == TargetPlatform.macOS)
-                            SemanticsAction.didGainAccessibilityFocus,
+                          if (defaultTargetPlatform == TargetPlatform.linux
+                              || defaultTargetPlatform == TargetPlatform.windows
+                              || defaultTargetPlatform == TargetPlatform.macOS)
+                            ...<SemanticsAction>[
+                              SemanticsAction.didGainAccessibilityFocus,
+                              SemanticsAction.didLoseAccessibilityFocus,
+                            ],
                         ],
                       ),
                     ],