chore(backend): standardize multiline formatting across modules

Reformatted multiline function calls, object definitions, and queries for improved code readability and consistency. Adjusted imports and constraints where necessary.

backend/app/schemas/syndarix/issue.py

@@ -99,9 +99,7 @@ class IssueAssign(BaseModel):
     def validate_assignment(self) -> "IssueAssign":
         """Ensure only one type of assignee is set."""
         if self.assigned_agent_id and self.human_assignee:
-            raise ValueError(
-                "Cannot assign to both an agent and a human. Choose one."
-            )
+            raise ValueError("Cannot assign to both an agent and a human. Choose one.")
         return self