[mle] refactor MleRouter and Mle classes into a single Mle class (#11411)

This commit refactors the `Mle` modules and combines the `MleRouter`
and `Mle` classes into a single `Mle` class which now handles both
FTD and MTD functionalities.

The `MleRouter` and `Mle` classes were originally intended as
sub-classes, where the base class `Mle` would provide MTD and common
behaviors, and `MleRouter` would implement FTD-specific behaviors.
However, over the years and as new features were implemented, these
two classes became more intertwined, and the `Mle` class began to
include many FTD-related functions and interactions with `MleRouter`
private variables and methods.

This commit simplifies the code by combining the two into a single
class. The previous `mle_router.cpp` file is also renamed to
`mle_ftd.cpp` to indicate that it implements FTD-specific MLE
behaviors.

tests/nexus/platform/nexus_node.cpp

@@ -40,7 +40,7 @@ void Node::Form(void)
     Get<MeshCoP::ActiveDatasetManager>().SaveLocal(datasetInfo);
 
     Get<ThreadNetif>().Up();
-    SuccessOrQuit(Get<Mle::MleRouter>().Start());
+    SuccessOrQuit(Get<Mle::Mle>().Start());
 }
 
 void Node::Join(Node &aNode, JoinMode aJoinMode)
@@ -51,7 +51,7 @@ void Node::Join(Node &aNode, JoinMode aJoinMode)
     switch (aJoinMode)
     {
     case kAsFed:
-        SuccessOrQuit(Get<Mle::MleRouter>().SetRouterEligible(false));
+        SuccessOrQuit(Get<Mle::Mle>().SetRouterEligible(false));
         OT_FALL_THROUGH;
 
     case kAsFtd:
@@ -72,7 +72,7 @@ void Node::Join(Node &aNode, JoinMode aJoinMode)
     Get<MeshCoP::ActiveDatasetManager>().SaveLocal(dataset);
 
     Get<ThreadNetif>().Up();
-    SuccessOrQuit(Get<Mle::MleRouter>().Start());
+    SuccessOrQuit(Get<Mle::Mle>().Start());
 }
 
 void Node::AllowList(Node &aNode)

tests/nexus/test_border_agent.cpp

@@ -866,7 +866,7 @@ void TestBorderAgentMeshCoPServiceChangedCallback(void)
     VerifyOrQuit(meshCoPServiceTester.FindTxtEntry("sb", txtEntry));
     VerifyOrQuit(meshCoPServiceTester.FindTxtEntry("pt", txtEntry));
     VerifyOrQuit(CheckObjectSameAsTxtEntryData(
-        txtEntry, BigEndian::HostSwap32(node0.Get<Mle::MleRouter>().GetLeaderData().GetPartitionId())));
+        txtEntry, BigEndian::HostSwap32(node0.Get<Mle::Mle>().GetLeaderData().GetPartitionId())));
     VerifyOrQuit(meshCoPServiceTester.FindTxtEntry("at", txtEntry));
     VerifyOrQuit(CheckObjectSameAsTxtEntryData(txtEntry, node0.Get<ActiveDatasetManager>().GetTimestamp()));
 

tests/nexus/test_dtls.cpp

@@ -247,12 +247,12 @@ void TestDtlsSingleSession(void)
     nexus.AdvanceTime(50 * Time::kOneSecondInMsec);
     VerifyOrQuit(node0.Get<Mle::Mle>().IsLeader());
 
-    SuccessOrQuit(node1.Get<Mle::MleRouter>().SetRouterEligible(false));
+    SuccessOrQuit(node1.Get<Mle::Mle>().SetRouterEligible(false));
     node1.Join(node0);
     nexus.AdvanceTime(20 * Time::kOneSecondInMsec);
     VerifyOrQuit(node1.Get<Mle::Mle>().IsChild());
 
-    SuccessOrQuit(node2.Get<Mle::MleRouter>().SetRouterEligible(false));
+    SuccessOrQuit(node2.Get<Mle::Mle>().SetRouterEligible(false));
     node2.Join(node0);
     nexus.AdvanceTime(20 * Time::kOneSecondInMsec);
     VerifyOrQuit(node2.Get<Mle::Mle>().IsChild());
@@ -489,12 +489,12 @@ void TestDtlsMultiSession(void)
     nexus.AdvanceTime(50 * Time::kOneSecondInMsec);
     VerifyOrQuit(node0.Get<Mle::Mle>().IsLeader());
 
-    SuccessOrQuit(node1.Get<Mle::MleRouter>().SetRouterEligible(false));
+    SuccessOrQuit(node1.Get<Mle::Mle>().SetRouterEligible(false));
     node1.Join(node0);
     nexus.AdvanceTime(20 * Time::kOneSecondInMsec);
     VerifyOrQuit(node1.Get<Mle::Mle>().IsChild());
 
-    SuccessOrQuit(node2.Get<Mle::MleRouter>().SetRouterEligible(false));
+    SuccessOrQuit(node2.Get<Mle::Mle>().SetRouterEligible(false));
     node2.Join(node0);
     nexus.AdvanceTime(20 * Time::kOneSecondInMsec);
     VerifyOrQuit(node2.Get<Mle::Mle>().IsChild());

tests/unit/test_child.cpp

@@ -78,7 +78,7 @@ void VerifyChildIp6Addresses(const Child &aChild, uint8_t aAddressListLength, co
     {
         Ip6::Address address;
 
-        if (sInstance->Get<Mle::MleRouter>().IsMeshLocalAddress(aAddressList[index]))
+        if (sInstance->Get<Mle::Mle>().IsMeshLocalAddress(aAddressList[index]))
         {
             SuccessOrQuit(aChild.GetMeshLocalIp6Address(address));
             VerifyOrQuit(address == aAddressList[index], "GetMeshLocalIp6Address() did not return expected address");
@@ -127,7 +127,7 @@ void TestChildIp6Address(void)
     numAddresses = 0;
 
     // First addresses uses the mesh local prefix (mesh-local address).
-    addresses[numAddresses] = sInstance->Get<Mle::MleRouter>().GetMeshLocalEid();
+    addresses[numAddresses] = sInstance->Get<Mle::Mle>().GetMeshLocalEid();
     addresses[numAddresses].SetIid(meshLocalIid);
 
     numAddresses++;

tests/unit/test_lowpan.cpp

@@ -104,7 +104,7 @@ static void Init(void)
     otMeshLocalPrefix meshLocalPrefix = {{0xfd, 0x00, 0xca, 0xfe, 0xfa, 0xce, 0x12, 0x34}};
     OffsetRange       offsetRange;
 
-    sInstance->Get<Mle::MleRouter>().SetMeshLocalPrefix(static_cast<Ip6::NetworkPrefix &>(meshLocalPrefix));
+    sInstance->Get<Mle::Mle>().SetMeshLocalPrefix(static_cast<Ip6::NetworkPrefix &>(meshLocalPrefix));
 
     // Emulate global prefixes with contextes.
     uint8_t mockNetworkData[] = {