Fix Date picker overlay colors aren't applied on selected state (flutter#159203)

## Description

This PR fixes the DatePicker overlay colors for the selected days.
Before this PR, overlays were obscured by the selected day backgound.
This fix simply replaces a DecoratedBox with an Ink to make the overlays
visible.
Combined with flutter#159072 which fixes
InkWell overlay color resolution related to the selected state, this PR
fixes [Date picker overlay colors aren't applied on
MaterialState.selected
state](flutter#130586).

Before, no overlay visible for the selected day when hovered, focused,
or pressed:


https://github.com/user-attachments/assets/944d5035-68b2-40da-b606-3e8795229767

After, overlay is visible for the selected day when hovered, focused, or
pressed (color change is slight as defined with M3 defaults):


https://github.com/user-attachments/assets/2627955b-f45a-465f-8eb0-21955ccd8c3e


## Related Issue

Fixes [Date picker overlay colors aren't applied on
MaterialState.selected
state](flutter#130586).

## Tests

Adds 12 tests.
Updates several existing tests (those tests were looking for a
`DecoratedBox`, make them look for an `Ink`).

Author: bleroux

examples/api/test/material/date_picker/date_picker_theme_day_shape.0_test.dart

@@ -14,41 +14,38 @@ void main() {
     const Color todayForegroundColor = Colors.black;
     const BorderSide todayBorder = BorderSide(width: 2);
 
+    ShapeDecoration? findDayDecoration(WidgetTester tester, String day) {
+      return tester.widget<Ink>(
+        find.ancestor(
+          of: find.text(day),
+          matching: find.byType(Ink)
+        ),
+      ).decoration as ShapeDecoration?;
+    }
+
     await tester.pumpWidget(
       const example.DatePickerApp(),
     );
 
     await tester.tap(find.text('Open Date Picker'));
     await tester.pumpAndSettle();
 
-    ShapeDecoration dayShapeDecoration = tester.widget<DecoratedBox>(find.ancestor(
-      of: find.text('15'),
-      matching: find.byType(DecoratedBox),
-    )).decoration as ShapeDecoration;
-
     // Test the current day shape decoration.
+    ShapeDecoration dayShapeDecoration = findDayDecoration(tester, '15')!;
     expect(dayShapeDecoration.color, todayBackgroundColor);
     expect(dayShapeDecoration.shape, dayShape.copyWith(side: todayBorder.copyWith(color: todayForegroundColor)));
 
-    dayShapeDecoration = tester.widget<DecoratedBox>(find.ancestor(
-      of: find.text('20'),
-      matching: find.byType(DecoratedBox),
-    )).decoration as ShapeDecoration;
-
     // Test the selected day shape decoration.
+    dayShapeDecoration = findDayDecoration(tester, '20')!;
     expect(dayShapeDecoration.color, theme.colorScheme.primary);
     expect(dayShapeDecoration.shape, dayShape);
 
     // Tap to select current day as the selected day.
     await tester.tap(find.text('15'));
     await tester.pumpAndSettle();
 
-    dayShapeDecoration = tester.widget<DecoratedBox>(find.ancestor(
-      of: find.text('15'),
-      matching: find.byType(DecoratedBox),
-    )).decoration as ShapeDecoration;
-
     // Test the selected day shape decoration.
+    dayShapeDecoration = findDayDecoration(tester, '15')!;
     expect(dayShapeDecoration.color, todayBackgroundColor);
     expect(dayShapeDecoration.shape, dayShape.copyWith(side: todayBorder.copyWith(color: todayForegroundColor)));
   });

packages/flutter/lib/src/material/calendar_date_picker.dart

@@ -20,6 +20,7 @@ import 'debug.dart';
 import 'divider.dart';
 import 'icon_button.dart';
 import 'icons.dart';
+import 'ink_decoration.dart';
 import 'ink_well.dart';
 import 'material_localizations.dart';
 import 'material_state.dart';
@@ -1056,7 +1057,7 @@ class _Day extends StatefulWidget {
   final bool isSelectedDay;
   final bool isToday;
   final ValueChanged<DateTime> onChanged;
-  final FocusNode? focusNode;
+  final FocusNode focusNode;
 
   @override
   State<_Day> createState() => _DayState();
@@ -1111,7 +1112,7 @@ class _DayState extends State<_Day> {
           shape: dayShape,
         );
 
-    Widget dayWidget = DecoratedBox(
+    Widget dayWidget = Ink(
       decoration: decoration,
       child: Center(
         child: Text(localizations.formatDecimal(widget.day.day), style: dayStyle?.apply(color: dayForegroundColor)),

packages/flutter/test/material/calendar_date_picker_test.dart

@@ -1196,7 +1196,7 @@ void main() {
       await gesture.moveTo(tester.getCenter(find.text('25')));
       await tester.pumpAndSettle();
       inkFeatures = tester.allRenderObjects.firstWhere((RenderObject object) => object.runtimeType.toString() == '_RenderInkFeatures');
-      expect(inkFeatures, paints..circle(radius: 35.0, color: theme.colorScheme.onSurfaceVariant.withOpacity(0.08)));
+      expect(inkFeatures, paints..circle()..circle(radius: 35.0, color: theme.colorScheme.onSurfaceVariant.withOpacity(0.08)));
       expect(inkFeatures, paintsExactlyCountTimes(#clipPath, 1));
 
       final Rect expectedClipRect = Rect.fromCircle(center: const Offset(400.0, 241.0), radius: 35.0);