chore(rstest-example): avoid logging undefined metadata in mocking example

[9aoy]

Summary

• avoid passing undefined metadata to console.debug/info/warn/error in the rstest/mocking logger example
• keep log output clean when callers only provide a message

Why

The example printed output like [Test] Test message undefined because the logger always forwarded the optional metadata argument as a second console parameter, even when it was not provided.

Impact

This keeps the example focused on the mocking behavior and removes confusing noise from test output.

Validation

• pnpm test in rstest/mocking