bug(sidenav): Sidenav opening animation triggers on page navigation

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

When using Angular Material’s Sidenav with `mode="side"` and `opened="true"`, navigating between pages triggers the opening animation, even though the sidenav is already in the open state. This behavior is inconsistent with the initial navigation, where the animation does not occur (which is the expected behavior).

### Reproduction

Steps to reproduce:
1. Go to https://material.angular.dev/components/sidenav/overview
2. Click on any link and go back to the previous page

### Expected Behavior

If the sidenav’s opened input is set to true initially, the opening animation should not play when moving between pages.

### Actual Behavior

The opening animation is triggered on page navigation, even if the sidenav is already open.

### Environment

- Angular: 21.2.6
- CDK/Material: 21.2.6
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

Beginne mit der Angular Material Sidenav-Implementierung und reproduziere das Verhalten aus den Schritten des Issues auf material.angular.dev mit mode="side" und opened="true". Verfolge, was während der Seitennavigation nach dem initialen Laden passiert, und überprüfe anschließend, dass die Navigation nicht mehr die Öffnungsanimation auslöst, während die Sidenav geöffnet bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.