Read failed conversion diagnostics from both output streams, retaining
raw bounded excerpts, observed byte totals, exit status and truncation.
Remove only repeated path prefixes from display text and report a useful
reason when the command prints no diagnostic. Reuse native failure
capture bounds rather than expanding accepted nonzero output into errors.
Adapted from PR #4851. Fixes#4849.
Validation: six controlled boundary failures on baseline, including the
stdout-only empty-reason defect; final46 passed across6files, real macOS
CLI/MCP controls, static checks, docs5 and discovery1565. On macOS26.3
plutil uses stderr; stdout-only coverage is controlled output. Separate
selected-target classification defect is reproduced and tracked as#4923.
Co-authored-by: apitoagents <339136651+apitoagents@users.noreply.github.com>
