[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.

Start by reviewing the unary logging tests added in pull request 3403 and identify how the showcase test uses TestAppender. Extend coverage to server-side streaming, client-side streaming, and bidirectional streaming calls, then verify logging behavior for each case. If pursuing the refactor, confirm that console log capture permits GrpcLoggingInterceptor to remain non-public.

Written by the indexing model from the issue text.