[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/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++;