fix: correct Manifest/ManifestIndex usage and add missing type annotations in tests

- test_docker_launch_committed_image: replace Manifest.from_json_obj
  (nonexistent) with ManifestIndex.from_json_obj; pass manifest= arg
  to DockerBottlePlan constructor (required by BottlePlan base class)
- test_macos_container_launch: cast SimpleNamespace stubs to their
  expected types (BottleSpec, GitGatePlan, EgressPlan) in _build_plan;
  add str type annotations to fake_build parameter signatures
- test_macos_container_util: add str type annotations to fake_build_image
  parameter signatures

tests/unit/test_macos_container_util.py

@@ -79,7 +79,7 @@ resolver #2
         )
         dockerfile_text = ""
 
-        def fake_build_image(image_tag, context, *, dockerfile=""):
+        def fake_build_image(image_tag: str, context: str, *, dockerfile: str = "") -> None:
             nonlocal dockerfile_text
             with open(dockerfile, encoding="utf-8") as f:
                 dockerfile_text = f.read()