[nexus] fix intermittent failure in test 6.1.2 (#12472)

This commit fixes an intermittent failure in Nexus test 6.1.2 by
setting the REED's router upgrade threshold to 0.

By default, a REED may attempt to upgrade to a Router if the number of
active routers in the network is below a certain threshold. In this
test scenario, we want to ensure the REED remains a child until the
appropriate point in the test sequence. Setting the threshold to 0
prevents premature promotion due to low router count, ensuring more
deterministic test behavior.

tests/nexus/test_6_1_2.cpp

@@ -115,6 +115,7 @@ void RunTest6_1_2(Topology aTopology, const char *aJsonFile)
     VerifyOrQuit(leader.Get<Mle::Mle>().IsLeader());
 
     reed.Join(leader);
+    reed.Get<Mle::Mle>().SetRouterUpgradeThreshold(0);
     nexus.AdvanceTime(kReedToRouterTime);
     VerifyOrQuit(reed.Get<Mle::Mle>().IsChild());