Summary

• Introduces LimitedString() — a factory function that creates graphene.String fields with min_length / max_length constraints. Constraint metadata is stored on the field and automatically propagated to BaseInputObjectType subclasses via _string_constraints.
• Adds automatic validation in BaseMutation.mutate() and DeprecatedModelMutation that recursively validates all LimitedString constraints before perform_mutation runs — no manual validation needed in individual mutations.
• Constraint descriptions are auto-appended to the GraphQL schema (e.g., "(maximum 250 characters.)")
• Replaces manual string length validation across several mutations:
  • Product media: alt field (max 250)
  • Payment transactions: card name (max 256), brand (max 40), firstDigits/lastDigits (max 4), other payment method name (max 256)
  • Order bulk create: note message (max 255)

Test plan

• Unit tests for LimitedString field factory (constraints, descriptions, assertions)
• Unit tests for BaseInputObjectType._string_constraints propagation and inheritance
• Unit tests for validate_string_constraints validator
• Unit tests for BaseMutation._validate_input_string_constraints (recursive, nested, list)
• Existing mutation tests updated to reflect new validation behavior

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