test: update macos-container tests for exec-tar commit approach

- Rename export test to reflect new exec-tar mechanism; update argv
  assertions to match the new `container exec ... tar` command shape
- Change mock stderr from str to bytes (subprocess.PIPE without text=True)
- Add type annotation to capture_freeze closure to satisfy pyright

tests/unit/test_backend_freezer.py

@@ -97,7 +97,7 @@ class TestFreezerBaseCommit(_FakeHomeMixin, unittest.TestCase):
         freezer = get_freezer("docker")
         captured = {}
 
-        def capture_freeze(agent):
+        def capture_freeze(agent: ActiveAgent) -> str:
             captured["slug"] = agent.slug
             return "some-ref"