ProtoSchemaConverter renders invalid schema for oneof in unwrap mode

### Describe the bug, including details regarding any error messages, version, and platform.

When unwrap is enabled all fields of the `TestProto3.OneOfTestMessage` will be required.

```
message TestProto3.OneOfTestMessage {
required int32 first = 1;
required int32 second = 2;
}
```
https://github.com/apache/parquet-java/blob/73a4430af6c40f8eb246ad4911eb6d103c9a2abe/parquet-protobuf/src/test/resources/TestProto3.proto#L116

This will never work but tests are missing for unwrap of `TestProto3.OneOfTestMessage`

The required repetition is added here:
https://github.com/apache/parquet-java/blob/73a4430af6c40f8eb246ad4911eb6d103c9a2abe/parquet-protobuf/src/main/java/org/apache/parquet/proto/ProtoSchemaConverter.java#L278

This could be caught early by adding `withValidation(true)` here:
https://github.com/apache/parquet-java/blob/73a4430af6c40f8eb246ad4911eb6d103c9a2abe/parquet-protobuf/src/test/java/org/apache/parquet/proto/TestUtils.java#L221

If validation is disabled it will fail when trying to read one of the non-existing fields in the `oneof`.

Is there really a need for setting all primitive fields in unwrap to required?

### Component(s)

_No response_

parquet-protobuf/src/main/java/org/apache/parquet/proto/ProtoSchemaConverter.java の unwrap repetition の処理から始め、続いて TestProto3.OneOfTestMessage fixture とそのテストを確認します。parquet-protobuf/src/test/java/org/apache/parquet/proto/TestUtils.java で validation を有効にし、unwrap mode のカバレッジを追加します。oneof conversion が有効な schema を生成し、関連する read path がテストされれば完了です。

索引モデルが issue の本文から書いたものです。