Grey out non-selectable days in CupertinoDatePicker (#136181)

This PR changes the text color of non-selectable days in the CupertinoDatePicker to match non-selectable months and years.

Current:
![Screenshot_1696829290](https://github.com/flutter/flutter/assets/147121557/209fee88-9efc-4b92-803a-453ecc6a7c16)

New:
![Screenshot_1696830232](https://github.com/flutter/flutter/assets/147121557/ecd11266-4c22-49cc-9bb5-2df39d10cf79)

Fixes #136179

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

@@ -1236,11 +1236,18 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
         children: List<Widget>.generate(31, (int index) {
           final int day = index + 1;
           final  int? dayOfWeek = widget.showDayOfWeek ? DateTime(selectedYear, selectedMonth, day).weekday : null;
+          final bool isInvalidDay = (day > daysInCurrentMonth)
+                                 || (widget.minimumDate?.year  == selectedYear &&
+                                     widget.minimumDate!.month == selectedMonth &&
+                                     widget.minimumDate!.day   >  day)
+                                 || (widget.maximumDate?.year  == selectedYear &&
+                                     widget.maximumDate!.month == selectedMonth &&
+                                     widget.maximumDate!.day   <  day);
           return itemPositioningBuilder(
             context,
             Text(
               localizations.datePickerDayOfMonth(day, dayOfWeek),
-              style: _themeTextStyle(context, isValid: day <= daysInCurrentMonth),
+              style: _themeTextStyle(context, isValid: !isInvalidDay),
             ),
           );
         }),

packages/flutter/test/cupertino/date_picker_test.dart

@@ -745,6 +745,162 @@ void main() {
       }
     });
 
+    testWidgets(
+      'non-selectable dates are greyed out, '
+      'when minimum date is unconstrained',
+      (WidgetTester tester) async {
+        final DateTime maximum = DateTime(2018, 6, 15);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.date,
+                  maximumDate: maximum,
+                  onDateTimeChanged: (_) {},
+                  initialDateTime: DateTime(2018, 6, 15),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // unconstrained bounds are not affected.
+        expect(
+          tester.widget<Text>(find.text('14')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+
+        // the selected day is not affected.
+        expect(
+          tester.widget<Text>(find.text('15')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+
+        // out of bounds and should be greyed out.
+        expect(
+          tester.widget<Text>(find.text('16')).style!.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+      },
+    );
+
+    testWidgets(
+      'non-selectable dates are greyed out, '
+      'when maximum date is unconstrained',
+      (WidgetTester tester) async {
+        final DateTime minimum = DateTime(2018, 6, 15);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.date,
+                  minimumDate: minimum,
+                  onDateTimeChanged: (_) {},
+                  initialDateTime: DateTime(2018, 6, 15),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // out of bounds and should be greyed out.
+        expect(
+          tester.widget<Text>(find.text('14')).style!.color,
+          isSameColorAs(CupertinoColors.inactiveGray.color),
+        );
+
+        // the selected day is not affected.
+        expect(
+          tester.widget<Text>(find.text('15')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+
+        // unconstrained bounds are not affected.
+        expect(
+          tester.widget<Text>(find.text('16')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+      },
+    );
+
+    testWidgets(
+      'non-selectable dates are greyed out, '
+      'months should be taken into account when greying out days',
+      (WidgetTester tester) async {
+        final DateTime minimum = DateTime(2018, 5, 15);
+        final DateTime maximum = DateTime(2018, 7, 15);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.date,
+                  minimumDate: minimum,
+                  maximumDate: maximum,
+                  onDateTimeChanged: (_) {},
+                  initialDateTime: DateTime(2018, 6, 15),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // days of a different min/max month are not affected.
+        expect(
+          tester.widget<Text>(find.text('14')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+        expect(
+          tester.widget<Text>(find.text('16')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+      },
+    );
+
+    testWidgets(
+      'non-selectable dates are greyed out, '
+      'years should be taken into account when greying out days',
+      (WidgetTester tester) async {
+        final DateTime minimum = DateTime(2017, 6, 15);
+        final DateTime maximum = DateTime(2019, 6, 15);
+        await tester.pumpWidget(
+          CupertinoApp(
+            home: Center(
+              child: SizedBox(
+                height: 400.0,
+                width: 400.0,
+                child: CupertinoDatePicker(
+                  mode: CupertinoDatePickerMode.date,
+                  minimumDate: minimum,
+                  maximumDate: maximum,
+                  onDateTimeChanged: (_) {},
+                  initialDateTime: DateTime(2018, 6, 15),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // days of a different min/max year are not affected.
+        expect(
+          tester.widget<Text>(find.text('14')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+        expect(
+          tester.widget<Text>(find.text('16')).style!.color,
+          isNot(isSameColorAs(CupertinoColors.inactiveGray.color)),
+        );
+      },
+    );
+
     testWidgets('picker automatically scrolls away from invalid date on month change', (WidgetTester tester) async {
       late DateTime date;
       await tester.pumpWidget(