fix(tests/e2e): surface diagnose step Detail (subprocess stderr) in EIC smoke output

mc#687 / mc#424 root-cause finding: the smoke test was extracting only
the Go error string (e.g. "exec: ... executable file not found") but
ignoring the subprocess stderr captured in each diagnoseStep's Detail field.

Detail carries the vendor-truth signal — e.g.
  "AccessDeniedException: ... is not authorized to perform:
   ec2-instance-connect:OpenTunnel"
— which is exactly what was needed to root-cause the 21h mc#424 outage
in 5 minutes instead of 21 hours.

The fix extracts both error + detail from the first failing step and
includes detail on its own line in the failure message. The shell
conditional ${DIAG_DETAIL:+\n  detail (subprocess stderr): $DIAG_DETAIL}
only appends the extra line when detail is non-empty, keeping the
output clean for simple errors.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/e2e/test_staging_full_saas.sh

@@ -511,8 +511,14 @@ for wid in $WS_TO_CHECK; do
     ok "    $wid terminal-reachable (canvas terminal will work)"
   else
     DIAG_FAIL=$(echo "$DIAG_JSON" | python3 -c "import json,sys; d=json.load(sys.stdin); print(d.get('first_failure','unknown'))" 2>/dev/null || echo "unknown")
-    DIAG_DETAIL=$(echo "$DIAG_JSON" | python3 -c "import json,sys; d=json.load(sys.stdin); s=[x for x in d.get('steps',[]) if not x.get('ok')]; print(s[0].get('error','') if s else '')" 2>/dev/null || echo "")
-    fail "Workspace $wid terminal diagnose failed at step '$DIAG_FAIL': $DIAG_DETAIL — check tenant SG has tcp/22 from EIC endpoint SG (sg-0785d5c6138220523), EIC_ENDPOINT_SG_ID set in Railway, and EIC endpoint health"
+    # Extract both error (Go error string) and detail (subprocess stderr — vendor truth).
+    # detail carries subprocess stderr for EIC/ssh/tunnel failures, which is the
+    # actionable signal (e.g. "AccessDeniedException: ... is not authorized to perform
+    # ec2-instance-connect:OpenTunnel"). mc#687 / mc#424 root-cause finding.
+    DIAG_ERR=$(echo "$DIAG_JSON" | python3 -c "import json,sys; d=json.load(sys.stdin); s=[x for x in d.get('steps',[]) if not x.get('ok')]; print(s[0].get('error','') if s else '')" 2>/dev/null || echo "")
+    DIAG_DETAIL=$(echo "$DIAG_JSON" | python3 -c "import json,sys; d=json.load(sys.stdin); s=[x for x in d.get('steps',[]) if not x.get('ok')]; print(s[0].get('detail','') if s else '')" 2>/dev/null || echo "")
+    fail "Workspace $wid terminal diagnose failed at step '$DIAG_FAIL': $DIAG_ERR${DIAG_DETAIL:+
+    detail (subprocess stderr): $DIAG_DETAIL} — check tenant SG has tcp/22 from EIC endpoint SG (sg-0785d5c6138220523), EIC_ENDPOINT_SG_ID set in Railway, and EIC endpoint health"
   fi
 done