[sdk-platform-java] Add showcase testing for logging on stream cases

Basic logging testing are included in https://github.com/googleapis/sdk-platform-java/pull/3403 for unary calls.
Test cases are missing for server side streaming, client side streaming and bidi-streaming.

We need more test coverage for logging behaviors on other type of calls beside unary calls.

Also, consider refactor showcase test to not depend on a TestAppender, but rather capture log outputs from console. This will allow logging implementation classes like GrpcLoggingInterceptor to be not public.

首先查看 pull request 3403 中新增的 unary logging 测试，并确定 showcase 测试如何使用 TestAppender。将覆盖范围扩展到服务器端流式、客户端流式和双向流式调用，然后验证每种情况下的 logging 行为。如果继续进行重构，请确认控制台日志捕获允许 GrpcLoggingInterceptor 保持为非 public。

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