test(fleet): bound stale heartbeat observation (#1043)

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -62,6 +62,19 @@ env -0 > "${MOSAIC_HOME:?}/fleet/pane-environment"
 SHIM
 chmod +x "$FAKE_BIN/mosaic"
 
+# Freeze numeric epoch reads only when a test arm supplies an observation bound.
+# Formatting reads still use the real BusyBox/POSIX date implementation.
+cat > "$FAKE_BIN/date" <<'SHIM'
+#!/usr/bin/env bash
+set -euo pipefail
+if [ -n "${MOSAIC_TEST_FIXED_EPOCH:-}" ] && [ "${1:-}" = '+%s' ]; then
+  printf '%s\n' "$MOSAIC_TEST_FIXED_EPOCH"
+  exit 0
+fi
+exec /bin/date "$@"
+SHIM
+chmod +x "$FAKE_BIN/date"
+
 write_generated() {
   local home="$1"
   local agent="$2"
@@ -89,6 +102,7 @@ run_start() {
   local agent="$2"
   HOME="$home" PATH="$FAKE_BIN:$PATH" MOSAIC_TEST_TMUX_CALLS="$TMUX_CALLS" \
     MOSAIC_TEST_PANE_PID="${MOSAIC_TEST_PANE_PID:-}" \
+    MOSAIC_TEST_FIXED_EPOCH="${MOSAIC_TEST_FIXED_EPOCH:-}" \
     MOSAIC_TEST_HOME="$home" \
     MOSAIC_TEST_FLEET_OWNER=123e4567-e89b-12d3-a456-426614174000 \
     MOSAIC_HOME="$home" "$START" "$agent"
@@ -403,7 +417,11 @@ write_heartbeat_local "$HOME_NATIVE_STALE" "coder-native-stale"
 STALE_HB="$HOME_NATIVE_STALE/run/coder-native-stale.hb"
 printf 'ts=native\npid=1\nstatus=busy\nmodel=stale-model\n' > "$STALE_HB"
 touch -t 200001010000.00 "$STALE_HB.native"
-MOSAIC_TEST_PANE_PID=$$ run_start "$HOME_NATIVE_STALE" coder-native-stale
+# Hold the sidecar's observation epoch constant: assertion runtime must not age
+# a fresh-marker mutant into the stale state that this fixture must distinguish.
+STALE_OBSERVATION_EPOCH=$(date +%s)
+MOSAIC_TEST_FIXED_EPOCH="$STALE_OBSERVATION_EPOCH" \
+  MOSAIC_TEST_PANE_PID=$$ run_start "$HOME_NATIVE_STALE" coder-native-stale
 wait_for_sidecar_status "$STALE_HB"
 
 HOME_NATIVE_ABSENT="$ROOT/native-absent"