[mle] refactor previous router/leader role restoration (#11728)

This commit updates `RestorePrevRole()` to directly start the
`RouterRoleRestorer` instead of calling `BecomeRouter()`.

Consequently, `BecomeRouter()` is simplified by removing the logic for
handling the `kRoleDetached` state. The method now focuses on the
child-to-router transition by sending an Address Solicit message, and
its initial role validation is made more explicit.

This change ensures the logic for restoring a previous router/leader
role is separate from the child-to-router transition logic.

src/core/thread/mle.cpp

@@ -290,13 +290,17 @@ Error Mle::RestorePrevRole(void)
 
     Error error = kErrorFailed;
 
+    VerifyOrExit(IsDetached());
     VerifyOrExit(GetRloc16() != kInvalidRloc16);
 
     if (IsRouterRloc16(GetRloc16()))
     {
 #if OPENTHREAD_FTD
         VerifyOrExit(mLastSavedRole == kRoleRouter || mLastSavedRole == kRoleLeader);
-        error = BecomeRouter(ThreadStatusTlv::kTooFewRouters);
+        VerifyOrExit(IsRouterEligible());
+        Get<MeshForwarder>().SetRxOnWhenIdle(true);
+        mRouterRoleRestorer.Start(mLastSavedRole);
+        error = kErrorNone;
 #endif
         ExitNow();
     }

src/core/thread/mle_ftd.cpp

@@ -189,8 +189,21 @@ Error Mle::BecomeRouter(ThreadStatusTlv::Status aStatus)
 {
     Error error = kErrorNone;
 
-    VerifyOrExit(!IsDisabled(), error = kErrorInvalidState);
-    VerifyOrExit(!IsRouterOrLeader(), error = kErrorNone);
+    switch (mRole)
+    {
+    case kRoleChild:
+        break;
+
+    case kRoleDisabled:
+    case kRoleDetached:
+        error = kErrorInvalidState;
+        OT_FALL_THROUGH;
+
+    case kRoleRouter:
+    case kRoleLeader:
+        ExitNow();
+    }
+
     VerifyOrExit(IsRouterEligible(), error = kErrorNotCapable);
 
     LogInfo("Attempt to become router");
@@ -198,19 +211,7 @@ Error Mle::BecomeRouter(ThreadStatusTlv::Status aStatus)
     Get<MeshForwarder>().SetRxOnWhenIdle(true);
     mRouterRoleTransition.StopTimeout();
 
-    switch (mRole)
-    {
-    case kRoleDetached:
-        mRouterRoleRestorer.Start(mLastSavedRole);
-        break;
-
-    case kRoleChild:
-        SuccessOrExit(error = SendAddressSolicit(aStatus));
-        break;
-
-    default:
-        OT_ASSERT(false);
-    }
+    error = SendAddressSolicit(aStatus);
 
 exit:
     return error;