test(core): pin stdout-only stderr handling

Six cases over a fake tool on PATH, covering both halves of the contract: the
tool's own stderr has to reach the user, and rtk must not invent a stdout
message in its place.

The prettier case is the one that is easy to get wrong. It writes its report to
stderr and nothing to stdout even on a successful run, so a filter's empty-input
placeholder ("Error: prettier produced no output") would be printed over a run
that worked. An empty stdout from a command that printed nothing on stdout is
the correct answer.

Also pinned: stderr is not replayed onto stdout on top of being forwarded, a
genuinely silent command stays silent, forwarding is not golangci-specific, and
exit codes propagate -- including golangci-lint's exit 1, which means issues
were found and is reported without failing the build.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>