Revert "Remove --preview as a required argument for ruff server (#12053)" (#12196)

This reverts commit b28dc9a.

We're not ready to stabilize the server yet. There's some pending work
for the VS Code extension and documentation improvements.

This change is to unblock Ruff release.

Author: dhruvmanila

crates/ruff/src/commands/server.rs

@@ -4,7 +4,12 @@ use crate::ExitStatus;
 use anyhow::Result;
 use ruff_server::Server;
 
-pub(crate) fn run_server(_preview: bool, worker_threads: NonZeroUsize) -> Result<ExitStatus> {
+pub(crate) fn run_server(preview: bool, worker_threads: NonZeroUsize) -> Result<ExitStatus> {
+    if !preview {
+        tracing::error!("--preview needs to be provided as a command line argument while the server is still unstable.\nFor example: `ruff server --preview`");
+        return Ok(ExitStatus::Error);
+    }
+
     let server = Server::new(worker_threads)?;
 
     server.run().map(|()| ExitStatus::Success)