Add return type for deep_merge function

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: amaslenn

src/cloudai/util/__init__.py

@@ -90,7 +90,7 @@ def prepare_output_dir(path: Path) -> Optional[Path]:
         return None
 
 
-def deep_merge(a: dict, b: dict):
+def deep_merge(a: dict, b: dict) -> dict:
     result = a.copy()
     for key in b:
         if key in result: