fix(date-picker): prevent value changes when disabled or readonly

Prevent the date picker from responding to focus events or input changes
when the component is disabled or readonly. This ensures that disabled
date pickers cannot be interacted with via keyboard or mouse.

Fixes #3815

Author: adrianschmidt-bot

src/components/date-picker/date-picker.tsx

@@ -279,6 +279,9 @@ export class DatePicker {
     }
 
     private showCalendar(event) {
+        if (this.disabled || this.readonly) {
+            return;
+        }
         this.showPortal = true;
         const inputElement = this.textField.shadowRoot.querySelector('input');
         setTimeout(() => {
@@ -369,6 +372,10 @@ export class DatePicker {
     }
 
     private handleInputElementChange(event) {
+        if (this.disabled || this.readonly) {
+            event.stopPropagation();
+            return;
+        }
         if (event.detail === '') {
             this.clearValue();
         }