UNC and other DOS Device Paths unusable

The following logic:
https://github.com/bytecodealliance/cap-std/blob/72fdc307184a642eadef22864434109c01d01528/cap-primitives/src/windows/fs/get_path.rs#L18-L22

completely breaks DOS device UNC paths such as:
```
\\?\UNC\server\share\bar.exe
```
which would have to be translated to:
```
\\server\share\bar.exe
```
in addition to that, other DOS device paths such as:
```
\\?\Volume{b75e2c83-0000-0000-0000-602f00000000}\Test\Foo.txt
```
get completely ruined.

There also is a lack of `\\.\` handling. I'm also not sure why this gets stripped in the first place, `join` can handle these paths just fine.

从 cap-primitives/src/windows/fs/get_path.rs 的第 18–22 行开始，复现示例 DOS 设备路径的处理方式。追踪当前逻辑为何会去除或修改前缀，然后验证 UNC、卷和 \\.\ 路径是否按照 issue 中的描述得到保留或转换。

由索引模型根据 Issue 内容生成。