frr: bgpd Send Hold Timer expiry bypasses the FSM, leaving a phantom Established session or crashing bgpd with Graceful Restart

In FRRouting 10.1 and later, bgpd does not correctly close a BGP session when the Send Hold Timer (RFC 9687) expires, i.e. when a neighbor has stopped accepting data from the router for twice the hold time. The connection is closed, but bgpd still considers the session to be up. A BGP neighbor that keeps sending KEEPALIVEs but stops reading from its connection, at a time when the router has a large amount of data to send to it (for example while sending it a full routing table), can trigger this. If the neighbor announced Graceful Restart support, bgpd crashes shortly after the session is closed, taking down all BGP sessions on the router, and the neighbor can repeat this each time bgpd restarts. Otherwise, the session stays stuck in the Established state: the neighbor's routes stay installed and keep being advertised to other peers, and the neighbor cannot reconnect until an operator clears the session manually. The neighbor does not need to be malicious; an overloaded or stuck peer can trigger the same behavior. No special configuration is required: any established BGP neighbor can trigger the issue, and Graceful Restart helper mode, which leads to the crash, is enabled by default.