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)
```

Reproduce the duplicate-operationId example with Reader::readFromJson, then trace the OpenApi::validate entry point and the validation it delegates to. Check how operations are collected and how uniqueness is represented. Done means the same example returns false, with coverage for duplicate operationIds.

Scritto dal modello di indicizzazione a partire dal testo della issue.