[Enhancement]: LocalFilesystemWithShell.execute — bound capture during drain, Future-based teardown, dual-stream test

[zzz-ghost]

Follow-ups from the review of #2839 (which fixed the pipe deadlock by draining stdout/stderr concurrently with waitFor):

1. Bound memory while draining, not after. maxOutputBytes is applied post-hoc to the assembled string, so a huge output can OOM before truncation runs. Cap buf.write once the buffer hits maxOutputBytes, keep reading-and-discarding so the OS pipe stays drained, and mark the result truncated when bytes were dropped.

2. Align reader teardown with ShellCommandTool. join(timeout) may return while the drainer thread is still alive, and ByteArrayOutputStream is not thread-safe, so toString() can race with write. Prefer Future.get(timeout) + cancel(true) (discarding the untrusted buffer), or interrupt the drainer when it is still alive after join.

3. Add a stderr / dual-stream regression test alongside the stdout one added in fix(harness): prevent pipe deadlock in LocalFilesystemWithShell.execute #2839.

Happy to send the PR(s) once #2839 is merged.