refactor: add field to APIError struct

internal/handler/response.go

@@ -23,6 +23,8 @@ type APIError struct {
 	Message string `json:"message"`
 	// Additional error details
 	Details string `json:"details,omitempty"`
+	// Field name if applicable
+	Field string `json:"field,omitempty" example:"name"`
 }
 
 const content_type = "Content-Type"