fix(kokoros): default optional status fields (#12447)

The speaker encoder metadata field makes the Rust status initializer
incomplete. Use protobuf defaults for absent metadata and memory fields.

Assisted-by: Codex:gpt-6

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

backend/rust/kokoros/src/service.rs

@@ -284,7 +284,7 @@ impl Backend for KokorosService {
         };
         Ok(Response::new(backend::StatusResponse {
             state,
-            memory: None,
+            ..Default::default()
         }))
     }