test_l3.py: Added comment for existing_station_list skip logic

VERIFIED CLI :
1. For running Multicast with real clients
python3 test_l3.py --lfmgr 192.168.207.78 --test_duration 1m --polling_interval 5s --upstream_port eth1 --endp_type mc_udp --side_b_min_bps=10000000 --tos BE --real

2. For running layer 3 traffic with real clients
python3 test_l3.py --lfmgr 192.168.207.78 --test_duration 1m --polling_interval 5s --upstream_port eth1 --endp_type lf_udp --side_b_min_bps=10000000 --tos BE --real

3. For running Multicast with both Existing and Virtual Stations
python3 test_l3.py --lfmgr 192.168.207.78 --endp_type mc_udp --tos BK --upstream_port eth1 --radio 'radio==wiphy1 stations==2 ssid==NETGEAR-2 ssid_pw==Password@123 security==wpa2' --test_duration 1m --polling_interval 5s --side_b_min_bps 102400000 --sta_start_offset 2000 --use_existing_station_list --existing_station_list 1.1.sta0000,1.1.sta0001

Author: Durga-CT

py-scripts/test_l3.py

@@ -2122,6 +2122,10 @@ def build(self, rebuild=False):
                             etype, self.side_b, tos=_tos, add_tos_to_name=True)
             self.mtx_endps = self.multicast_profile.get_mc_names()
             logger.info("Creating test station port(s)")
+            # For real + virtual or existing + virtual combinations, the first
+            # station profile represents the real/existing stations. Only the
+            # remaining profiles are used to create virtual stations, so skip
+            # station creation for i == 0 when using existing station lists.
             for i, station_profile in enumerate(self.station_profiles):
                 if not rebuild and (not self.use_existing_station_lists or self.use_existing_station_lists and i != 0):
                     station_profile.use_security(