[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        | -
@webmozart Then you would need to check `$this->context instanceof ExecutionContext` in RecursiveContextualValidator whenever you call setNode. This is pretty ugly. So I think changing the interface is cleaner.