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

Angular Material Sidenav の実装から始め、mode="side" と opened="true" を使用して、material.angular.dev にある issue の手順の動作を再現します。初回読み込み後のページ遷移中に何が起きるかを追跡し、その後、sidenav が開いたままの状態で、ページ遷移によって開くアニメーションが発生しなくなることを確認します。

索引モデルが issue の本文から書いたものです。