Cannot build testbed on Windows

https://github.com/huggingface/llm-ls/blob/2a433cdf75dc0a225e95753256f2601161bc6747/crates/testbed/src/main.rs#L346C24-L346C24

The linked statement results in the following error.

```text
error[E0425]: cannot find function `symlink` in module `fs`
--> crates\testbed\src\main.rs:346:21
|
346 | fs::symlink(link_target, dst_path.clone()).await?;
| ^^^^^^^ not found in `fs`
|
note: found an item that was configured out
--> C:\Users\noahw\.cargo\registry\src\index.crates.io-6f17d22bba15001f\tokio-1.32.0\src\fs\mod.rs:114:9
|
114 | mod symlink;
| ^^^^^^^
note: found an item that was configured out
--> C:\Users\noahw\.cargo\registry\src\index.crates.io-6f17d22bba15001f\tokio-1.32.0\src\fs\mod.rs:115:28
|
115 | pub use self::symlink::symlink;
|
```

This is available in unix only
https://docs.rs/tokio/latest/tokio/fs/fn.symlink.html

Start at crates/testbed/src/main.rs:346 and reproduce the Windows build error shown in the issue. Read Tokio's filesystem symlink documentation and the surrounding testbed setup to understand the platform limitation. Done means the testbed builds successfully on Windows without breaking the existing Unix behavior.

Written by the indexing model from the issue text.