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

从 Angular Material Sidenav 实现开始，使用 mode="side" 和 opened="true"，复现 issue 在 material.angular.dev 步骤中描述的行为。跟踪初始加载后页面导航期间发生的情况，然后验证在 sidenav 保持打开的同时，导航不再触发打开动画。

由索引模型根据 Issue 内容生成。