"Truncate at cap and append the ellipsis" admits both 501 and 500 total
length; the two would desync test assertions from the telemetry length
promise. Cap is now the total including the marker.
Issue #10: the 400 body dies in _status_to_error, and telemetry only
writes str(exc), so adding a field alone would not make the refusal
queryable after the fact. Design keeps the summary in both the message
and a new base-class body_text, across every non-2xx branch and both
transports.
iomgaa