feat(list-header): add inner part (#30930)

Issue number: N/A

---------

## What is the current behavior?
The inner structural element of list-header is not exposed as a shadow part, preventing users from being able to customize its styles directly.

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

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

---------

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

core/api.txt

@@ -1033,6 +1033,7 @@ ion-list-header,css-prop,--color,ios
 ion-list-header,css-prop,--color,md
 ion-list-header,css-prop,--inner-border-width,ios
 ion-list-header,css-prop,--inner-border-width,md
+ion-list-header,part,inner
 
 ion-loading,scoped
 ion-loading,prop,animated,boolean,true,false,false

core/src/components/list-header/list-header.tsx

@@ -7,6 +7,8 @@ import type { Color } from '../../interface';
 
 /**
  * @virtualProp {"ios" | "md"} mode - The mode determines which platform styles to use.
+ *
+ * @part inner - The inner wrapper element that arranges the list header content.
  */
 @Component({
   tag: 'ion-list-header',
@@ -40,7 +42,7 @@ export class ListHeader implements ComponentInterface {
           [`list-header-lines-${lines}`]: lines !== undefined,
         })}
       >
-        <div class="list-header-inner">
+        <div class="list-header-inner" part="inner">
           <slot></slot>
         </div>
       </Host>

core/src/components/list-header/test/custom/list-header.e2e.ts

@@ -0,0 +1,34 @@
+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('list-header: custom'), () => {
+    test.describe('CSS shadow parts', () => {
+      test('should be able to customize inner part', async ({ page }) => {
+        await page.setContent(
+          `
+          <style>
+            ion-list-header::part(inner) {
+              background-color: red;
+            }
+          </style>
+
+          <ion-list-header>Header</ion-list-header>
+        `,
+          config
+        );
+
+        const header = page.locator('ion-list-header');
+        const backgroundColor = await header.evaluate((el) => {
+          const shadowRoot = el.shadowRoot;
+          const inner = shadowRoot?.querySelector('.list-header-inner');
+          return inner ? window.getComputedStyle(inner).backgroundColor : '';
+        });
+        expect(backgroundColor).toBe('rgb(255, 0, 0)');
+      });
+    });
+  });
+});