Duplicate OperationIds should be considered invalid

## Expected
Calling `validate` on a `\cebe\openapi\spec\OpenApi` object containing duplicate operationIds will return false.
### Reasoning
[OperationIds MUST be unique among all operations described in the API.](https://spec.openapis.org/oas/v3.1.0#operation-object)

## Actual
Calling `validate` on a `\cebe\openapi\spec\OpenApi` object containing duplicate operationIds returns true.

### Example

```php
$api = Reader::readFromJson(<<validate()); // bool(true)
```