Allow empty payloads in Redis and MinIO adapters.

Relax write validation so {} and zero-byte BytesIO round-trip correctly,
document None-vs-empty semantics on interfaces and adapters, and add
integration tests for placeholders and existence distinction.

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

python_repositories/interfaces/object_repository_interface.py

@@ -11,8 +11,10 @@ class ObjectRepositoryInterface(ABC):
     def get(self, object_name: str) -> BytesIO | None:
         """Get an object by name.
 
-        Returns None when the object does not exist. Raises ConnectionError when
-        not connected. Other backend errors propagate to the caller.
+        Returns ``None`` when the object does not exist. Returns an empty
+        ``BytesIO`` for a zero-byte object. Use ``value is not None`` to test
+        existence. Raises ConnectionError when not connected. Other backend
+        errors propagate to the caller.
         """
         ...
 
@@ -23,12 +25,17 @@ class ObjectRepositoryInterface(ABC):
         data: BytesIO,
         content_type: str = "application/octet-stream",
     ) -> None:
-        """Put an object by name."""
+        """Put an object by name.
+
+        Accepts zero-byte ``BytesIO``. A zero-byte object is returned by
+        ``get()`` as an empty buffer, not ``None``. Use ``delete()`` to remove
+        an object entirely.
+        """
         ...
 
     @abstractmethod
     def delete(self, object_name: str) -> None:
-        """Delete an object by name."""
+        """Delete an object by name, removing it entirely."""
         ...
 
     @abstractmethod