feat(item-divider): add inner and container parts (#30928)

Issue number: N/A

---------

## What is the current behavior?
The inner structural elements of item-divider are not exposed as shadow parts, preventing users from being able to customize their styles directly.

## What is the new behavior?
- Exposes `inner` and `container` shadow parts
- Adds e2e test coverage for customizing the shadow parts

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

---------

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

core/api.txt

@@ -968,6 +968,8 @@ ion-item-divider,css-prop,--padding-start,ios
 ion-item-divider,css-prop,--padding-start,md
 ion-item-divider,css-prop,--padding-top,ios
 ion-item-divider,css-prop,--padding-top,md
+ion-item-divider,part,container
+ion-item-divider,part,inner
 
 ion-item-group,none
 

core/src/components/item-divider/item-divider.tsx

@@ -11,6 +11,9 @@ import type { Color } from '../../interface';
  * @slot - Content is placed between the named slots if provided without a slot.
  * @slot start - Content is placed to the left of the divider text in LTR, and to the right in RTL.
  * @slot end - Content is placed to the right of the divider text in LTR, and to the left in RTL.
+ *
+ * @part inner - The inner wrapper element that arranges the divider content.
+ * @part container - The wrapper element that contains the default slot.
  */
 @Component({
   tag: 'ion-item-divider',
@@ -50,8 +53,8 @@ export class ItemDivider implements ComponentInterface {
         })}
       >
         <slot name="start"></slot>
-        <div class="item-divider-inner">
-          <div class="item-divider-wrapper">
+        <div class="item-divider-inner" part="inner">
+          <div class="item-divider-wrapper" part="container">
             <slot></slot>
           </div>
           <slot name="end"></slot>

core/src/components/item-divider/test/custom/item-divider.e2e.ts

@@ -0,0 +1,57 @@
+import { expect } from '@playwright/test';
+import { configs, test } from '@utils/test/playwright';
+
+/**
+ * This behavior does not vary across modes/directions
+ */
+configs({ directions: ['ltr'], modes: ['md'] }).forEach(({ title, config }) => {
+  test.describe(title('item-divider: custom'), () => {
+    test.describe('CSS shadow parts', () => {
+      test('should be able to customize inner part', async ({ page }) => {
+        await page.setContent(
+          `
+          <style>
+            ion-item-divider::part(inner) {
+              background-color: red;
+            }
+          </style>
+
+          <ion-item-divider>Divider</ion-item-divider>
+        `,
+          config
+        );
+
+        const divider = page.locator('ion-item-divider');
+        const backgroundColor = await divider.evaluate((el) => {
+          const shadowRoot = el.shadowRoot;
+          const inner = shadowRoot?.querySelector('.item-divider-inner');
+          return inner ? window.getComputedStyle(inner).backgroundColor : '';
+        });
+        expect(backgroundColor).toBe('rgb(255, 0, 0)');
+      });
+
+      test('should be able to customize container part', async ({ page }) => {
+        await page.setContent(
+          `
+          <style>
+            ion-item-divider::part(container) {
+              background-color: green;
+            }
+          </style>
+
+          <ion-item-divider>Divider</ion-item-divider>
+        `,
+          config
+        );
+
+        const divider = page.locator('ion-item-divider');
+        const backgroundColor = await divider.evaluate((el) => {
+          const shadowRoot = el.shadowRoot;
+          const container = shadowRoot?.querySelector('.item-divider-wrapper');
+          return container ? window.getComputedStyle(container).backgroundColor : '';
+        });
+        expect(backgroundColor).toBe('rgb(0, 128, 0)');
+      });
+    });
+  });
+});