fix arp proxy v6 issue (#20964)

Description of PR
Summary:
Fix the following error on t0-isolated-d96u32s2 topo.

arp/test_arp_extended.py::test_proxy_arp[v6-str5-7060x6-moby-512-1-None]
-------------------------------- live log call ---------------------------------
03/10/2025 02:10:49 __init__.pytest_runtest_call             L0040 ERROR  | Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/_pytest/python.py", line 1788, in runtest
    self.ihook.pytest_pyfunc_call(pyfuncitem=self)
  File "/usr/local/lib/python3.8/dist-packages/pluggy/_hooks.py", line 513, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
  File "/usr/local/lib/python3.8/dist-packages/pluggy/_manager.py", line 120, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/usr/local/lib/python3.8/dist-packages/pluggy/_callers.py", line 139, in _multicall
    raise exception.with_traceback(exception.__traceback__)
  File "/usr/local/lib/python3.8/dist-packages/pluggy/_callers.py", line 103, in _multicall
    res = hook_impl.function(*args)
  File "/usr/local/lib/python3.8/dist-packages/_pytest/python.py", line 194, in pytest_pyfunc_call
    result = testfunction(**testargs)
  File "/var/src/sonic-mgmt_vms91-t0-7060x6-moby-512-1/tests/arp/test_arp_extended.py", line 98, in test_proxy_arp
    testutils.verify_packet(ptfadapter, expected_packet, ptf_intf_index, timeout=10)
  File "/usr/local/lib/python3.8/dist-packages/ptf/testutils.py", line 3250, in verify_packet
    test.fail(
  File "/usr/lib/python3.8/unittest/case.py", line 753, in fail
    raise self.failureException(msg)
AssertionError: Expected packet was not received on device 0, port 32.
When ndppd is starting up, it will read the whole ipv6 routes via /proc/net/ipv6_route. For topology with large ipv6 routes and next hops, this reading may take long time to finish.

Type of change
 Bug fix
 Testbed and Framework(new/improvement)
 New Test case
 Skipped for non-supported platforms
 Test case improvement

Approach
What is the motivation for this PR?
Fix test_arp_extended.py::test_proxy_arp failure for large topo.

How did you do it?
Add an extra delay in test case by reading /proc/net/ipv6_route to match up with the behavior in ndppd.

How did you verify/test it?
physical testbed.

signed-off-by: jianquanye@microsoft.com
Signed-off-by: selldinesh <dinesh.sellappan@keysight.com>

Author: sdszhang

tests/arp/test_arp_extended.py

@@ -87,6 +87,12 @@ def test_proxy_arp(rand_selected_dut, proxy_arp_enabled, ip_and_intf_info, ptfad
         ndppd_conf = rand_selected_dut.shell('docker exec swss cat /etc/ndppd.conf',
                                              module_ignore_errors=True)['stdout']
         logger.debug(ndppd_conf)
+        # when there are a large number of routes, ndppd will take long time to read /proc/net/ipv6_route.
+        # instead of sleep for a specific time, we just log the time taken to read the file to match the delay time.
+        # once ndppd performance is improved, this can be removed.
+        ipv6_routes_read_time = rand_selected_dut.shell("docker exec swss bash -c 'time wc -l /proc/net/ipv6_route'")
+        logger.debug("Total ipv6 route entries: {} \n Read time:{}".format(ipv6_routes_read_time['stdout'],
+                                                                           ipv6_routes_read_time['stderr']))
 
         neigh_table = rand_selected_dut.shell('ip -6 neigh')['stdout']
         logger.debug(neigh_table)