real-multi-data-input.ga has step-level 'optional' field not in native schema

The fixture `gxformat2/examples/native/real-multi-data-input.ga` (origin: real) has an `"optional"` key at the step level (steps 0 and 1) that is not part of the `NativeStep` schema. This causes strict Pydantic validation to fail.

```json
"0": {
"optional": true,
"type": "data_input",
...
}
```

Unclear where this field came from — it's not a standard Galaxy native workflow export field. Options:
- If Galaxy actually exports this, the native schema needs updating
- If it was manually added, the fixture origin should be `real-hacked` and the field removed or the schema updated

Discovered while adding comprehensive schema validation declarative tests for all catalog fixtures.