Add version mismatch check to SetState matching Go's State.Update

Go's State.Update compares the incoming version with the stored
version and returns "version mismatch" error if they differ. This
provides optimistic concurrency control. The Rust implementation
was accepting any version unconditionally.

seaweed-volume/src/server/grpc_server.rs

@@ -852,8 +852,15 @@ impl VolumeServer for VolumeGrpcService {
         let req = request.into_inner();
 
         if let Some(new_state) = &req.state {
-            // Match Go: simply apply the state without version checking.
-            // Go's SetState calls vs.store.State.Update(req.GetState()) unconditionally.
+            // Go's State.Update checks version: if incoming version != stored version → error.
+            let current_version = self.state.state_version.load(Ordering::Relaxed);
+            if new_state.version != current_version {
+                return Err(Status::failed_precondition(format!(
+                    "version mismatch for VolumeServerState (got {}, want {})",
+                    new_state.version, current_version
+                )));
+            }
+
             self.state
                 .maintenance
                 .store(new_state.maintenance, Ordering::Relaxed);