fix: correctly support !Send Actix APIs in server functions

[gbj]

Actix pins requests and responses to a single thread, allowing it to use a number of !Send types in its ecosystem. You need to be able to hold these across .await points.

Leptos server functions and resources want server functions to be Send.

The #[server] macro currently includes a workaround for Actix that automatically wraps a server fn body in SendWrapper in the trait implementation. However, it didn't do this in the dummy function body, with the result that this didn't actually work—it would work in the trait implementation but not the dummy function. (The dummy function only needs to exist for better rust-analyzer support.)

Test case:

#[server]
pub async fn list_post_metadata() -> Result<Vec<PostMetadata>, ServerFnError> {
    // really these would be some Actix ecosystem type that's an Rc internally
    let unsend = std::rc::Rc::new(());
    tokio::time::sleep(std::time::Duration::from_secs(1)).await;
    let _unsend = unsend.clone();
    Ok(POSTS
        .iter()
        .map(|data| PostMetadata {
            id: data.id,
            title: data.title.clone(),
        })
        .collect())
}