feat(datetime): add border property to highlightedDates (#30534)

Issue number: resolves #29833

---------

- Adds the `border` property for customizing the border of highlighted dates
- Adds the `border` to the e2e test for highlightedDates
- Updates screenshots

---------

Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>

core/src/components/datetime/datetime-interface.ts

@@ -22,15 +22,15 @@ export type DatetimePresentation = 'date-time' | 'time-date' | 'date' | 'time' |
 
 export type TitleSelectedDatesFormatter = (selectedDates: string[]) => string;
 
-export type DatetimeHighlightStyle =
-  | {
-      textColor: string;
-      backgroundColor?: string;
-    }
-  | {
-      textColor?: string;
-      backgroundColor: string;
-    };
+/**
+ * DatetimeHighlightStyle must include textColor, backgroundColor, or border.
+ * It cannot be an empty object.
+ */
+export type DatetimeHighlightStyle = {
+  textColor?: string;
+  backgroundColor?: string;
+  border?: string;
+} & ({ textColor: string } | { backgroundColor: string } | { border: string });
 
 export type DatetimeHighlight = { date: string } & DatetimeHighlightStyle;