fix(backup): re-raise inspect failures for containers that still exist

Treating every failed swarm-task inspect as skippable opened a false-green
window: a transient inspect failure on a still-running, non-whitelisted
container skipped the stop and backed the volume up hot while the run
reported success. Re-check whether the container is still listed; only a
genuinely vanished container skips, an existing one re-raises so a broken
daemon keeps failing the run loudly. Covered by unit tests for both paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/backup/test_docker_swarm.py

@@ -21,7 +21,7 @@ class TestIsSwarmTask(unittest.TestCase):
     @patch.object(
         docker_mod,
         "execute_shell_command",
-        side_effect=BackupException("gone"),
+        side_effect=[BackupException("gone"), []],
     )
     def test_vanished_container_counts_as_not_stoppable(self, _mock) -> None:
         # A container removed between listing and inspect must not abort the
@@ -29,6 +29,18 @@ class TestIsSwarmTask(unittest.TestCase):
         # stop/start and image-inspect path.
         self.assertTrue(docker_mod.is_swarm_task("gone-container"))
 
+    @patch.object(
+        docker_mod,
+        "execute_shell_command",
+        side_effect=[BackupException("daemon hiccup"), ["still-here"]],
+    )
+    def test_inspect_failure_on_existing_container_still_fails(self, _mock) -> None:
+        # If the container still exists, an inspect failure must keep failing
+        # the run: silently skipping the stop would back up a hot volume and
+        # report green without the stop guarantee.
+        with self.assertRaises(BackupException):
+            docker_mod.is_swarm_task("still-here")
+
 
 class TestFilterStoppable(unittest.TestCase):
     @patch.object(docker_mod, "is_swarm_task", side_effect=[False, True, False])