Centralize message role invariants

internal/usecase/repairer.go

@@ -59,7 +59,7 @@ func (r *defaultOutputRepairer) Repair(ctx context.Context, req RepairRequest) (
 	copy(messages, req.OriginalMessages)
 	if strings.TrimSpace(req.PreviousOutput) != "" {
 		messages = append(messages, domain.RenderedMessage{
-			Role:    "assistant",
+			Role:    domain.RoleAssistant,
 			Content: req.PreviousOutput,
 		})
 	}
@@ -69,7 +69,7 @@ func (r *defaultOutputRepairer) Repair(ctx context.Context, req RepairRequest) (
 		previousResponse = "The previous response is included immediately before this instruction."
 	}
 	messages = append(messages, domain.RenderedMessage{
-		Role: "user",
+		Role: domain.RoleUser,
 		Content: fmt.Sprintf(
 			"Repair attempt %d of %d for validation mode %s.\n"+
 				"Preserve valid values and change only what is necessary.\n"+