chaos: fail fast on missing RUNTIME argument

Without a RUNTIME arg, chaos_check.sh silently proceeds with an empty
value, which becomes --duration s in the chaos harness invocation and
fails 15+ seconds later — after the JDK/agent/jar setup — with a
NumberFormatException buried in the harness output, reported generically
as "FAIL:Chaos harness crashed (RC=1)". Check for a missing RUNTIME right
after arg parsing and fail immediately with a clear usage message.

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jbachorik

.gitlab/reliability/chaos_check.sh

@@ -9,6 +9,11 @@ RUNTIME=${1}
 CONFIG=${2:-profiler+tracer}
 ALLOCATOR=${3:-gmalloc}
 
+if [ -z "${RUNTIME}" ]; then
+  echo "FAIL:missing RUNTIME argument (usage: $0 <runtime-seconds> [config] [allocator])" >&2
+  exit 1
+fi
+
 echo "Chaos run: runtime=${RUNTIME}s config=${CONFIG} allocator=${ALLOCATOR}"
 
 CHAOS_JDK="${CHAOS_JDK:-21.0.3-tem}"