Validate instance states during wait and clarify region logic.

Add checks to ensure cluster states align with instance statuses during the wait operation, raising errors on mismatches. Enhance comments to explain how the region is inferred from project IDs due to the lack of direct region information retrieval in Nebius.

sky/provision/nebius/instance.py

@@ -143,8 +143,23 @@ def run_instances(region: str, cluster_name_on_cloud: str,
 
 def wait_instances(region: str, cluster_name_on_cloud: str,
                    state: Optional[status_lib.ClusterStatus]) -> None:
-    del state
     _wait_until_no_pending(region, cluster_name_on_cloud)
+    if state is not None:
+        if state == status_lib.ClusterStatus.UP:
+            stopped_instances = _filter_instances(region, cluster_name_on_cloud,
+                                                  ['STOPPED'])
+            if stopped_instances:
+                raise RuntimeError(
+                    f'Cluster {cluster_name_on_cloud} is in UP state, but '
+                    f'{len(stopped_instances)} instances are stopped.')
+        if state == status_lib.ClusterStatus.STOPPED:
+            running_instances = _filter_instances(region, cluster_name_on_cloud,
+                                                  ['RUNNIG'])
+
+            if running_instances:
+                raise RuntimeError(
+                    f'Cluster {cluster_name_on_cloud} is in STOPPED state, but '
+                    f'{len(running_instances)} instances are running.')
 
 
 def stop_instances(

sky/provision/nebius/utils.py

@@ -35,6 +35,13 @@ def get_project_by_region(region: str) -> str:
     service = nebius.iam().ProjectServiceClient(nebius.sdk())
     projects = service.list(nebius.iam().ListProjectsRequest(
         parent_id=nebius.get_tenant_id())).wait()
+    # To find a project in a specific region, we rely on the project ID to
+    # deduce the region, since there is currently no method to retrieve region
+    # information directly from the project. Additionally, there is only one
+    # project per region, and projects cannot be created at this time.
+    # The region is determined from the project ID using a region-specific
+    # identifier embedded in it.
+    # https://docs.nebius.com/overview/regions
     for project in projects.items:
         if region == 'eu-north1' and project.metadata.id[8:11] == 'e00':
             return project.metadata.id