[nexus] migrate test_ping_lla_src.py to nexus (#12990)

This commit migrates the test_ping_lla_src.py script from thread-cert
to the Nexus test framework.

The new test_ping_lla_src.cpp implements the same test logic:
- Forms a network with a Leader and two Routers.
- Verifies that pings using a Link-Local Address (LLA) as the source
  succeed when sent to a neighbor's Mesh-Local EID (ML-EID).
- Verifies that pings using an LLA source fail when sent to a
  non-neighbor's ML-EID, as LLAs are only valid for single-hop
  communication.
- Verifies that external routes are not used for LLA-sourced packets.

To support this migration, the Nexus Core class was enhanced with:
- Overloads for SendAndVerifyEchoRequest that allow specifying a
  source address.
- New SendAndVerifyNoEchoResponse methods to verify that no echo
  response is received (useful for negative test scenarios).

Changes:
- Added tests/nexus/test_ping_lla_src.cpp
- Updated tests/nexus/CMakeLists.txt to include the new test.
- Enhanced tests/nexus/platform/nexus_core.hpp/cpp with new helpers.
- Removed tests/scripts/thread-cert/test_ping_lla_src.py.

tests/scripts/thread-cert/test_ping_lla_src.py

@@ -1,119 +0,0 @@
-#!/usr/bin/env python3
-#
-#  Copyright (c) 2021, The OpenThread Authors.
-#  All rights reserved.
-#
-#  Redistribution and use in source and binary forms, with or without
-#  modification, are permitted provided that the following conditions are met:
-#  1. Redistributions of source code must retain the above copyright
-#     notice, this list of conditions and the following disclaimer.
-#  2. Redistributions in binary form must reproduce the above copyright
-#     notice, this list of conditions and the following disclaimer in the
-#     documentation and/or other materials provided with the distribution.
-#  3. Neither the name of the copyright holder nor the
-#     names of its contributors may be used to endorse or promote products
-#     derived from this software without specific prior written permission.
-#
-#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 'AS IS'
-#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
-#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-#  POSSIBILITY OF SUCH DAMAGE.
-#
-
-import logging
-import unittest
-
-import pktverify
-from pktverify import packet_verifier
-import config
-import thread_cert
-
-# Test description:
-# The purpose of this test is to verify the Router's routing behavior using LLA as source address.
-#
-# Topology:
-#
-#
-#  ROUTER_2 ----- ROUTER_1 ---- ROUTER_3
-#
-#
-
-ROUTER_1 = 1
-ROUTER_2 = 2
-ROUTER_3 = 3
-
-
-class TestPing(thread_cert.TestCase):
-    USE_MESSAGE_FACTORY = False
-    SUPPORT_NCP = False
-
-    TOPOLOGY = {
-        ROUTER_1: {
-            'name': 'Router_1',
-            'allowlist': [ROUTER_2, ROUTER_3],
-        },
-        ROUTER_2: {
-            'name': 'Router_2',
-            'allowlist': [ROUTER_1],
-        },
-        ROUTER_3: {
-            'name': 'Router_3',
-            'allowlist': [ROUTER_1],
-        },
-    }
-
-    def test(self):
-        router1 = self.nodes[ROUTER_1]
-        router2 = self.nodes[ROUTER_2]
-        router3 = self.nodes[ROUTER_3]
-
-        router1.start()
-        self.simulator.go(config.LEADER_STARTUP_DELAY)
-        self.assertEqual('leader', router1.get_state())
-
-        router2.start()
-        self.simulator.go(config.ROUTER_STARTUP_DELAY)
-        self.assertEqual('router', router2.get_state())
-
-        router3.start()
-        self.simulator.go(config.ROUTER_STARTUP_DELAY)
-        self.assertEqual('router', router3.get_state())
-
-        self.simulator.go(10)
-
-        left, center, right = router2, router1, router3
-
-        left_lla = left.get_ip6_address(config.ADDRESS_TYPE.LINK_LOCAL)
-
-        # Ping a neighbor with LLA src should succeed.
-        self.assertTrue(left.ping(center.get_mleid(), interface=left_lla))
-        self.simulator.go(3)
-
-        # Ping a non-neighbor with LLA src should fail.
-        self.assertFalse(left.ping(right.get_mleid(), interface=left_lla))
-        self.simulator.go(3)
-
-        center.add_route('2001::/64', stable=True)
-        center.register_netdata()
-
-        self.simulator.go(3)
-
-        # Make sure external routes are not used for LLA src.
-        self.assertFalse(left.ping("2001::1", interface=left_lla))
-        self.simulator.go(3)
-
-    def verify(self, pv: pktverify.packet_verifier.PacketVerifier):
-        pkts = pv.pkts
-        # Make sure the Ping Request to 2001::1 was not sent
-        pkts.filter_ipv6_dst('2001::1').filter_ping_request().must_not_next()
-
-
-if __name__ == '__main__':
-    unittest.main()