* fix(transport): suppress noisy WARN on stdio client shutdown
When Stop() kills the subprocess, cmd.Wait() always returns a non-zero
exit code which is expected behavior. Previously this propagated as an
error, causing "failed to stop stdio client ... exit status 1" warnings
on every normal CLI exit for stdio-based plugins.
Now Stop() returns nil when the process was explicitly killed, keeping
the error path only for cases where the process exits on its own with
a non-zero code (e.g. stdin close without kill).
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* fix(transport): address review feedback — simplify Stop(), fix test assertion
- Remove redundant `killed` flag; inline Kill+Wait+return nil
- Update integration test to positively assert Stop() returns nil after kill
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FuShu-Yang
649801e479