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_

从 parquet-protobuf/src/main/java/org/apache/parquet/proto/ProtoSchemaConverter.java 中 unwrap repetition 的处理开始，然后检查 TestProto3.OneOfTestMessage fixture 及其测试。在 parquet-protobuf/src/test/java/org/apache/parquet/proto/TestUtils.java 中启用 validation，并为 unwrap mode 添加 coverage。完成的标准是 oneof conversion 生成有效的 schema，并且相关的 read path 得到测试。

由索引模型根据 Issue 内容生成。