In QUIC BBRv2, use inflight byte after congestion event to detect queuing during PROBE_UP. Protected by quic_reloadable_flag_quic_bbr2_use_post_inflight_to_detect_queuing. PiperOrigin-RevId: 332262071 Change-Id: Ib057f75556f9b3bfe9873d5a538b938d6d38ba03