collab: Update how mode is displayed in root endpoint (#15911)

This PR adjusts how we display the "mode" collab is running in on the
root endpoint.

It's minor, but it does make things a bit cleaner.

Release Notes:

- N/A

crates/collab/src/lib.rs

@@ -235,7 +235,8 @@ impl Config {
 }
 
 /// The service mode that collab should run in.
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(Debug, PartialEq, Eq, Clone, Copy, strum::Display)]
+#[strum(serialize_all = "snake_case")]
 pub enum ServiceMode {
     Api,
     Collab,

crates/collab/src/main.rs

@@ -279,10 +279,7 @@ async fn setup_llm_database(config: &Config) -> Result<()> {
 }
 
 async fn handle_root(Extension(mode): Extension<ServiceMode>) -> String {
-    format!(
-        "collab {mode:?} v{VERSION} ({})",
-        REVISION.unwrap_or("unknown")
-    )
+    format!("zed:{mode} v{VERSION} ({})", REVISION.unwrap_or("unknown"))
 }
 
 async fn handle_liveness_probe(