fix(material/menu): prevent interaction with <a> in disabled menu items (#30203)

[IgnacioCabanellas]

Fix the issue where tags inside disabled mat-menu-items were still interactive, allowing users to navigate even when the menu item was disabled.

Problem:

• The anchor links inside disabled mat-menu-items were still interactive.
• Users could right click on them and navigate to the associated link with the Open link in new tab for example, despite the items being disabled.

Solution:

• Added the CSS rule pointer-events: none; to disable interaction with anchor tags inside disabled mat-menu-item elements.

Additional Information:

• A StackBlitz demo is available to showcase both the bug and the fix: https://stackblitz.com/~/github.com/IgnacioCabanellas/met-menu-item-demo

Fixes #30203

[crisbeto]

We intentionally removed the pointer-events: none in #19183 to avoid a different issue.