[Validator] Fixed metadata serialization

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Previously, when using a metadata cache, the information about the `traverse` and `deep` properties of the `Valid` constraint was lost upon unserialization. As a result, collection traversal was always skipped when the validation metadata was loaded from the cache, leading to potential security vulnerabilities.

Thanks to @alexandresalome for the report.