[mle] use compact Route TLV in Link Accept to child neighbors (#13012)

This commit enhances MLE where a full Route TLV could be appended to
a Link Accept message sent to a child neighbor, potentially leading
to a message requiring lowpan fragmentation.

Previously, `Mle::SendLinkAccept()` relied on a `Router` pointer to
determine whether to use a full or compact Route TLV. When the Link
Request originated from a child, this pointer was null, causing a
full Route TLV to be used.

The changes in this commit include:
- Updating the `LinkAcceptInfo` struct to track the RLOC16 of the Link
  Request sender.
- Updating `Mle::TxMessage::AppendRouteTlv()` and adding
  `AppendCompactRouteTlv()` to replace the previous single method that
  took a `Neighbor` pointer. This makes the intent clearer and
  supports both router and child neighbors.
- Updating `RouterTable::FillRouteTlv()` to take an RLOC16 instead of
  a `Neighbor` pointer. It uses `Mle::RouterIdFromRloc16()` to ensure
  that if the destination is a child, its parent's Router ID is
  included in the compact Route TLV.
- Includes new Nexus test `test_compact_route_tlv` to validate the
  use of compact Route TLV in Link Accept.

tests/nexus/CMakeLists.txt

@@ -394,6 +394,7 @@ ot_nexus_test(child_supervision "core;nexus")
 ot_nexus_test(coap_block "core;nexus")
 ot_nexus_test(coap_observe "core;nexus")
 ot_nexus_test(coaps "core;nexus")
+ot_nexus_test(compact_route_tlv "core;nexus")
 ot_nexus_test(dataset_updater "core;nexus")
 ot_nexus_test(discover_scan "core;nexus")
 ot_nexus_test(dnssd "core;nexus")

tests/nexus/test_compact_route_tlv.cpp

@@ -0,0 +1,136 @@
+/*
+ *  Copyright (c) 2026, 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.
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "platform/nexus_core.hpp"
+#include "platform/nexus_node.hpp"
+
+namespace ot {
+namespace Nexus {
+
+void TestCompactRouteTlv(void)
+{
+    static constexpr uint16_t kNumRouters = 31;
+
+    Core  nexus;
+    Node &leader = nexus.CreateNode();
+    Node *routers[kNumRouters];
+
+    Log("---------------------------------------------------------------------------------------");
+    Log("TestCompactRouteTlv");
+
+    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    Log("Form topology - leader + %u routers", kNumRouters);
+
+    leader.Get<Mle::Mle>().SetRouterUpgradeThreshold(32);
+    leader.Get<Mle::Mle>().SetRouterDowngradeThreshold(33);
+
+    leader.Form();
+    nexus.AdvanceTime(100 * Time::kOneSecondInMsec);
+    VerifyOrQuit(leader.Get<Mle::Mle>().IsLeader());
+
+    for (uint16_t i = 0; i < kNumRouters; i++)
+    {
+        Node &router = nexus.CreateNode();
+
+        routers[i] = &router;
+
+        router.Get<Mle::Mle>().SetRouterUpgradeThreshold(32);
+        router.Get<Mle::Mle>().SetRouterDowngradeThreshold(33);
+        router.Get<Mle::Mle>().SetRouterSelectionJitter(20);
+
+        router.Join(leader, Node::kAsFtd);
+        nexus.AdvanceTime(30 * Time::kOneSecondInMsec);
+
+        VerifyOrQuit(router.Get<Mle::Mle>().IsRouter());
+
+        Log("Router %-2u -> rloc16: 0x%04x", i, router.Get<Mle::Mle>().GetRloc16());
+    }
+
+    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    Log("Check Active Router Count on all routers");
+
+    nexus.AdvanceTime(300 * Time::kOneSecondInMsec);
+
+    VerifyOrQuit(leader.Get<RouterTable>().GetActiveRouterCount() == kNumRouters + 1);
+
+    for (const Node *router : routers)
+    {
+        VerifyOrQuit(router->Get<RouterTable>().GetActiveRouterCount() == kNumRouters + 1);
+    }
+
+    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    Log("Reset `routers[0]` and validate that it is restored quickly");
+
+    routers[0]->Reset();
+
+    routers[0]->Get<Mle::Mle>().SetRouterUpgradeThreshold(32);
+    routers[0]->Get<Mle::Mle>().SetRouterDowngradeThreshold(33);
+    routers[0]->Get<Mle::Mle>().SetRouterSelectionJitter(20);
+
+    routers[0]->Get<ThreadNetif>().Up();
+    SuccessOrQuit(routers[0]->Get<Mle::Mle>().Start());
+
+    for (uint16_t i = 0; i < 5000; i++)
+    {
+        nexus.AdvanceTime(1);
+
+        if (routers[0]->Get<Mle::Mle>().IsRouter())
+        {
+            Log("Node `routers[0]` restored it role after reset");
+            Log("Validate that it got a compact Route TLV and therefore does not yet see all routers");
+            VerifyOrQuit(routers[0]->Get<RouterTable>().GetActiveRouterCount() < kNumRouters + 1);
+            break;
+        }
+
+        VerifyOrQuit(routers[0]->Get<Mle::Mle>().IsDetached());
+    }
+
+    VerifyOrQuit(routers[0]->Get<Mle::Mle>().IsRouter());
+
+    Log("Validate that `routes[0]` discovered all routers");
+
+    nexus.AdvanceTime(100 * Time::kOneSecondInMsec);
+
+    VerifyOrQuit(routers[0]->Get<Mle::Mle>().IsRouter());
+    VerifyOrQuit(routers[0]->Get<RouterTable>().GetActiveRouterCount() == kNumRouters + 1);
+}
+
+} // namespace Nexus
+} // namespace ot
+
+int main(void)
+{
+    ot::Nexus::TestCompactRouteTlv();
+    printf("All tests passed\n");
+
+    return 0;
+}