[Validator] Made sure that context changes don't leak out of (Contextual)ValidatorInterface

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11046
| License       | MIT
| Doc PR        | -
@Tobion Not necessarily, as we're weakening the API here (which is allowed for subclasses). I don't want to change the interface in a patch release, if possible.

@hhamon The default value is only there to allow passing `null` as `$metadata`. Both arguments should be passed explicitly.