[br] add a new test for stale favored on-link prefix deprecation (#13620)

Adds a unit test `TestStaleFavoredOnLinkPrefix()` to validate that when
an external router advertises an on-link prefix that becomes favored,
its deprecation after the stale timer expiration triggers the border
router to start advertising its local on-link prefix.

tests/unit/test_routing_manager.cpp

@@ -2637,6 +2637,109 @@ void TestFavoredOnLinkPrefix(void)
     FinalizeTest();
 }
 
+void TestStaleFavoredOnLinkPrefix(void)
+{
+    Ip6::Prefix  localOnLink;
+    Ip6::Prefix  localOmr;
+    Ip6::Prefix  onLinkPrefix   = PrefixFromString("2000:abba:baba::", 64);
+    Ip6::Address routerAddressA = AddressFromString("fd00::aaaa");
+    uint16_t     heapAllocations;
+
+    Log("--------------------------------------------------------------------------------------------");
+    Log("TestStaleFavoredOnLinkPrefix");
+
+    InitTest();
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    // Start Routing Manager.
+
+    sRsEmitted   = false;
+    sRaValidated = false;
+    sExpectedPio = kNoPio;
+    sExpectedRios.Clear();
+
+    heapAllocations = sHeapAllocatedPtrs.GetLength();
+    SuccessOrQuit(sInstance->Get<BorderRouter::RoutingManager>().SetEnabled(true));
+
+    SuccessOrQuit(sInstance->Get<BorderRouter::RoutingManager>().GetOnLinkPrefix(localOnLink));
+    SuccessOrQuit(sInstance->Get<BorderRouter::RoutingManager>().GetOmrPrefix(localOmr));
+
+    Log("Local on-link prefix is %s", localOnLink.ToString().AsCString());
+    Log("Local OMR prefix is %s", localOmr.ToString().AsCString());
+
+    sExpectedRios.Add(localOmr);
+
+    // Advance time to allow the initial router discovery cycle to start
+    // and emit the first RS message (random start delay of up to 1 second
+    // followed by 4-second intervals between RS transmissions).
+
+    AdvanceTime(5000);
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    // Send an RA from router A advertising an on-link prefix with a longer valid
+    // lifetime and a shorter preferred lifetime. It should be chosen as the
+    // favored on-link prefix, preventing BR from advertising its local prefix.
+
+    SendRouterAdvert(routerAddressA, {Pio(onLinkPrefix, kValidLitime, kPreferredLifetime)});
+
+    AdvanceTime(30 * 1000);
+
+    VerifyOrQuit(sRsEmitted);
+    VerifyOrQuit(sRaValidated);
+    VerifyOrQuit(sExpectedRios.SawAll());
+
+    VerifyFavoredOnLinkPrefix(onLinkPrefix);
+    VerifyPrefixTable({OnLinkPrefix(onLinkPrefix, kValidLitime, kPreferredLifetime, routerAddressA)});
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    // Wait until just before the 600-second stale time expires. Confirm that the
+    // stale timer has not fired yet and no RS messages are sent.
+
+    sRsEmitted = false;
+
+    AdvanceTime(560 * 1000);
+
+    VerifyOrQuit(!sRsEmitted);
+    VerifyFavoredOnLinkPrefix(onLinkPrefix);
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    // Wait for the stale timer to expire. This starts RsSender to transmit RS
+    // messages to check if the router/prefix is still refreshed.
+
+    AdvanceTime(12 * 1000);
+
+    VerifyOrQuit(sRsEmitted);
+    VerifyFavoredOnLinkPrefix(onLinkPrefix);
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    // Allow RsSender to complete its RS transmission cycle with no RA response.
+    // Confirm that the prefix is deprecated and BR begins advertising its own
+    // local on-link prefix in emitted RAs.
+
+    sRaValidated = false;
+    sExpectedPio = kPioAdvertisingLocalOnLink;
+    sExpectedRios.Clear();
+    sExpectedRios.Add(localOmr);
+
+    AdvanceTime(30000);
+
+    VerifyOrQuit(sRaValidated);
+    VerifyOrQuit(sExpectedRios.SawAll());
+
+    VerifyFavoredOnLinkPrefix(localOnLink);
+    VerifyPrefixTable({OnLinkPrefix(onLinkPrefix, kValidLitime, 0, routerAddressA)});
+
+    //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+    SuccessOrQuit(sInstance->Get<BorderRouter::RoutingManager>().SetEnabled(false));
+    AdvanceTime(3000);
+
+    VerifyOrQuit(heapAllocations == sHeapAllocatedPtrs.GetLength());
+
+    Log("End of TestStaleFavoredOnLinkPrefix");
+    FinalizeTest();
+}
+
 void TestLocalOnLinkPrefixDeprecation(void)
 {
     static constexpr uint32_t kMaxRaTxInterval = 196; // In seconds
@@ -5741,6 +5844,7 @@ int main(void)
     ot::TestNonUlaPioWithOnlyOnLinkFlag();
     ot::TestAdvNonUlaRoute();
     ot::TestFavoredOnLinkPrefix();
+    ot::TestStaleFavoredOnLinkPrefix();
     ot::TestLocalOnLinkPrefixDeprecation();
     ot::TestUnadvertisedLocalOnLinkPrefix();
     ot::TestExtPanIdChange();