fix(demo-client): reset LAST_EVENT_ID on fresh task start

When reusing a session for a new task, the stale LAST_EVENT_ID from
the previous run caused the client to skip the first stage header.
Now resets to 0 when POST returns status='started'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RaviPidaparthi

sdk/agentserver/azure-ai-agentserver-invocations/samples/durable-agent-demo/demo-client.sh

@@ -199,6 +199,10 @@ dispatch_task() {
     if [[ -n "$sess" ]]; then
         SESSION_ID="$sess"
     fi
+    # Reset event tracking when a fresh task starts (not a reconnect)
+    if [[ "$status" == "started" ]]; then
+        LAST_EVENT_ID="0"
+    fi
     save_session
 
     echo -e "${DIM}Task ${status}: inv=${INV_ID:0:30}...${RESET}"