feat: Enhance ID mapping service robustness, add defensive guards, and expand migration engine and API testing.

backend/src/services/git_service.py

@@ -51,6 +51,8 @@ class GitService:
     # @RETURN: str
     def _get_repo_path(self, dashboard_id: int) -> str:
         with belief_scope("GitService._get_repo_path"):
+            if dashboard_id is None:
+                raise ValueError("dashboard_id cannot be None")
             return os.path.join(self.base_path, str(dashboard_id))
     # [/DEF:_get_repo_path:Function]