fix(material/checkbox): account for disabledInteractive in harness

Switches to using a CSS class to get the disabled state in the harness so it continues to work when `disabledInteractive` is set.

(cherry picked from commit 56b977f)

src/material/checkbox/testing/checkbox-harness.spec.ts

@@ -167,6 +167,17 @@ describe('MatCheckboxHarness', () => {
     await disabledCheckbox.toggle();
     expect(await disabledCheckbox.isChecked()).toBe(false);
   });
+
+  it('should get disabled state for checkbox with disabledInteractive', async () => {
+    fixture.componentInstance.disabled.set(false);
+    fixture.componentInstance.disabledInteractive.set(true);
+
+    const checkbox = await loader.getHarness(MatCheckboxHarness.with({label: 'Second'}));
+    expect(await checkbox.isDisabled()).toBe(false);
+
+    fixture.componentInstance.disabled.set(true);
+    expect(await checkbox.isDisabled()).toBe(true);
+  });
 });
 
 @Component({
@@ -179,7 +190,11 @@ describe('MatCheckboxHarness', () => {
         aria-label="First checkbox">
       First
     </mat-checkbox>
-    <mat-checkbox indeterminate="true" [disabled]="disabled()" aria-labelledby="second-label">
+    <mat-checkbox
+      indeterminate="true"
+      [disabled]="disabled()"
+      aria-labelledby="second-label"
+      [disabledInteractive]="disabledInteractive()">
       Second
     </mat-checkbox>
     <span id="second-label">Second checkbox</span>
@@ -190,4 +205,5 @@ describe('MatCheckboxHarness', () => {
 class CheckboxHarnessTest {
   ctrl = new FormControl(true);
   disabled = signal(true);
+  disabledInteractive = signal(false);
 }

src/material/checkbox/testing/checkbox-harness.ts

@@ -72,8 +72,14 @@ export class MatCheckboxHarness extends ComponentHarness {
 
   /** Whether the checkbox is disabled. */
   async isDisabled(): Promise<boolean> {
-    const disabled = (await this._input()).getAttribute('disabled');
-    return coerceBooleanProperty(await disabled);
+    const input = await this._input();
+    const disabled = await input.getAttribute('disabled');
+
+    if (disabled !== null) {
+      return coerceBooleanProperty(disabled);
+    }
+
+    return (await input.getAttribute('aria-disabled')) === 'true';
   }
 
   /** Whether the checkbox is required. */