fix: fix arp_update complains missing interface entry for static route (sonic-net#21513)

Fix

E               2025 Nov 26 22:08:52.060655 ERR swss2#arp_update[16275]: 178 missing interface entry for static route 20.1.1.1

What is the motivation for this PR?
For tear down, we need to remove the route first before the arp. Otherwise swss will throw error

How did you do it?
How did you verify/test it?
MS Only: https://elastictest.org/scheduler/testplan/692f7c4c8a84f286cc565304

Signed-off-by: Austin Pham <[email protected]>

Author: auspham

tests/common/snappi_tests/snappi_fixtures.py

@@ -1515,6 +1515,7 @@ def gen_data_flow_dest_ip(addr, dut=None, intf=None, namespace=None, setup=True)
     if setup:
         cmd = "add"
     asic_arg = ""
+
     if namespace is not None:
         asic_arg = f"ip netns exec {namespace}"
     int_arg = ""
@@ -1526,11 +1527,18 @@ def gen_data_flow_dest_ip(addr, dut=None, intf=None, namespace=None, setup=True)
         arp_opt = f"-d {addr}"
 
     try:
-        dut.shell(f"sudo {asic_arg} arp {int_arg} {arp_opt}")
-        dut.shell(
-            "{} config route {} prefix {}/32 nexthop {} {}".format(
-                asic_arg, cmd, DEST_TO_GATEWAY_MAP[addr]['dest'], addr,
-                DEST_TO_GATEWAY_MAP[addr]['intf']))
+        if setup:
+            dut.shell(f"sudo {asic_arg} arp {int_arg} {arp_opt}")
+            dut.shell(
+                f"{asic_arg} config route {cmd} prefix {DEST_TO_GATEWAY_MAP[addr]['dest']}/32 nexthop "
+                f"{addr} {DEST_TO_GATEWAY_MAP[addr]['intf']}"
+            )
+        else:
+            dut.shell(
+                f"{asic_arg} config route {cmd} prefix {DEST_TO_GATEWAY_MAP[addr]['dest']}/32 nexthop "
+                f"{addr} {DEST_TO_GATEWAY_MAP[addr]['intf']}"
+            )
+            dut.shell(f"sudo {asic_arg} arp {int_arg} {arp_opt}")
     except RunAnsibleModuleFail:
         if setup:
             raise