Java: When calling getContent or getItems, get null if the other property was set.

In Java, when creating a ChatMessageContent, you can either set the content or items of the object. If you then retrieve either, the system isn't able to smartly pull from the right source.

For example, if I set content to "hello world" and I call getItems(), I should receive an array of KernelContent with a single TextContent inside of it. The reverse should also be true.

定位 Java ChatMessageContent 类，并检查 getContent() 和 getItems() 以及 content 和 items 的存储方式。重现 issue 中描述的两种访问顺序，然后为每个转换方向添加覆盖。完成的标准是：设置 content 后，getItems() 会暴露一个 TextContent；设置 items 后，getContent() 会返回对应的内容。