[Test] Add comprehensive resize unit and smoke tests

Unit tests (17 total, 13 new):
- Scale-up is noop in pre-provision
- Same-size is noop
- Scale-down rejects RUNNING, SETTING_UP, and PENDING jobs
- Scale-down error message lists all in-progress job IDs
- Scale-down aborts on SSH failure (safety guard)
- Scale-down succeeds on idle cluster (terminates workers)
- Scale-down allows completed/failed/cancelled jobs
- Scale-down to 1 node (head-only)
- _check_existing_cluster uses task.num_nodes when resize=True
- _check_existing_cluster uses handle.launched_nodes when resize=False
- _check_existing_cluster warns (not errors) for non-existent cluster
- CLI --resize without -c raises UsageError

Smoke tests:
- Add test_resize_cli_validation: --resize without -c errors,
  --resize on non-existent cluster warns and creates normally

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Hao Zhang

tests/smoke_tests/test_cluster_job.py

@@ -3540,3 +3540,24 @@ def test_resize(generic_cloud: str):
         timeout=10 * 60,
     )
     smoke_tests_utils.run_one_test(test)
+
+
+@pytest.mark.kubernetes
+def test_resize_cli_validation(generic_cloud: str):
+    """Test resize CLI validation: --resize without -c should error."""
+    name = smoke_tests_utils.get_cluster_name()
+    test = smoke_tests_utils.Test(
+        'resize_cli_validation',
+        [
+            # --resize without -c should error.
+            'sky launch --resize --num-nodes 4 2>&1 && '
+            'exit 1 || echo "Correctly rejected"',
+            # --resize on a non-existent cluster should warn and create.
+            f'sky launch -y -c {name} --resize --infra kubernetes '
+            f'--cpus 2 --num-nodes 1',
+            f's=$(sky status {name}) && echo "$s" && echo "$s" | grep {name} | grep UP',
+        ],
+        f'sky down -y {name}',
+        timeout=5 * 60,
+    )
+    smoke_tests_utils.run_one_test(test)