required should be inferred from the return type of a hybrid property

With a definition like:
```
@hybrid_property
def is_active(self) -> bool:
return self.state == 'ACTIVE'
```

The resulting graphql field type is currently marked `Boolean` unless overridden by `ORMField()`. Column properties correctly determine the nullability from the column definition. It seems reasonable to do the same for the hybrid_property based on the declared type. If the return type were `bool | None`, then it would be optional, otherwise it should be required.

先追蹤 hybrid_property 欄位如何轉換成 GraphQL 欄位，然後將這條路徑與現有的 column property 可空性推論進行比較。當宣告的 bool 回傳型別產生必要的 Boolean 欄位，而 bool | None 產生選用欄位，且兩種情況都有 coverage 時，變更即完成。

由索引模型根據 Issue 內容生成。