fix(datetime): selected today button renders correctly on ios (#28740)

Issue number: Internal

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

We removed the background color from today's calendar day button if
selected when implementing the calendar-day button shadow part feature:
79b005da70 (diff-8e2912d52559aa44c9c6dc062c8d683e7b51c92c7b7ba420ad6f7587f1e1a61aL139)

We did not catch this because we do not have test coverage for this use
case. We typically avoid rendering datetime on today's date because
"today" could be something totally different depending on the test
machine's locale/configuration.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Adds the background rule that was removed
- Added test coverage. I mocked today's date to be a fixed date so it
never changes

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

---------

Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/datetime/datetime.ios.scss

@@ -249,9 +249,11 @@
 
 /**
  * Day that is selected and is today
- * should have white color.
+ * should have base background color
+ * with contrast text.
  */
 :host .calendar-day.calendar-day-today.calendar-day-active {
+  background: current-color(base);
   color: current-color(contrast);
 }
 

core/src/components/datetime/test/basic/datetime.e2e.ts

@@ -458,6 +458,44 @@ configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, config }) => {
   });
 });
 
+configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
+  test.describe(title('datetime: today button rendering'), () => {
+    test('should render today button correctly when selected', async ({ page }) => {
+      test.info().annotations.push({
+        type: 'issue',
+        description: 'FW-5808',
+      });
+      await page.setContent(
+        `
+        <ion-datetime presentation="date" value="2022-01-02"></ion-datetime>
+
+        <script>
+          const mockToday = '2022-01-02T16:22';
+          Date = class extends Date {
+            constructor(...args) {
+              if (args.length === 0) {
+                super(mockToday)
+              } else {
+                super(...args);
+              }
+            }
+          }
+        </script>
+      `,
+        config
+      );
+
+      const datetime = page.locator('ion-datetime');
+
+      await page.waitForSelector('.datetime-ready');
+
+      await expect(datetime.locator('.calendar-day-today')).toHaveScreenshot(
+        screenshot(`datetime-today-calendar-button`)
+      );
+    });
+  });
+});
+
 /**
  * The calendar day highlight does not render
  * on iOS and has the same behavior across directions.