test(row253): expose peer failures and pin reasons (mos-1297)

The peer-output fixture execs isolated system Python to write its exact bytes
and exit with peer-status. This avoids masking parser/status faults behind the
supervisor session check. All17 faults assert an exact reason with a delimiter;
every existing assertion remains. No product change or timing delay.

Old packages/cli/tests/case1-accept.test.mjs:100:
const PYTHON_STUB = `[ -f "$HC/peer-output" ] && { cat "$HC/peer-output"; exit "$(cat "$HC/peer-status")"; }

Diagnostic under sage-2752 found probe-inner-session for malformed output and
nonzero exit. Mos confirmed the fixture mask and authorized this repair.
No tests ran after the repair; Duckworth review and fresh whole gate follow.
