ValidationState with composition keywords to cache branch selection and avoid re-resolving it at unmarshal time.

• Remove duplicate schema validation during unmarshalling by introducing an internal trusted ValidationState path and reusing it through nested/composed schemas.
• Add regression coverage to confirm request and response body schemas are validated only once during unmarshal.
• Extend multipart form deserialization to correctly resolve oneOf/anyOf branches with typed form fields, including preserving encoding metadata and normalizing multipart form values before casting.