Move structural typing tests into interface unit test files.

Align test layout with the one-test-file-per-interface convention by removing structural_typing_test.py.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/object_repository_interface_test.py

@@ -9,6 +9,32 @@ from python_repositories.interfaces.object_repository_interface import (
 )
 
 
+class InMemoryObjectRepo:
+    """Plain class that satisfies ObjectRepositoryInterface without inheritance."""
+
+    def get(self, object_name: str) -> BytesIO | None:
+        return None
+
+    def put(
+        self,
+        object_name: str,
+        data: BytesIO,
+        content_type: str = "application/octet-stream",
+    ) -> None:
+        pass
+
+    def delete(self, object_name: str) -> None:
+        pass
+
+    def list_objects(self, prefix: str = "") -> list[str]:
+        return []
+
+
+def accepts_object_repo(repo: ObjectRepositoryInterface) -> None:
+    """Type-checking hook for ObjectRepositoryInterface structural subtyping."""
+    repo.get("object")
+
+
 def test_instantiation_fails_when_get_not_implemented() -> None:
     """Test that instantiation fails if get is not implemented."""
 
@@ -98,3 +124,10 @@ def test_instantiation_fails_when_list_objects_not_implemented() -> None:
 
     with pytest.raises(TypeError):
         _ = Incomplete()  # type: ignore
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies ObjectRepositoryInterface structurally."""
+    repo: ObjectRepositoryInterface = InMemoryObjectRepo()
+    accepts_object_repo(repo)
+    assert isinstance(repo, ObjectRepositoryInterface)