[mle] disable MLE retransmissions when detaching (#9929)

When detaching, a child transmits a Child Update Request message with
Timeout TLV set to 0. The child waits for 1 second after sending the
Child Update Request message before stopping Thread
operation. However, during this time, the MLE retransmission timer may
fire and retransmit the Child Update Request message, which can cause
it to send a new Child Update Request message with a non-zero Timeout
TLV.

This commit update the behavior to avoid retransmitting the Child
Update Request message when detaching.

src/core/thread/mle.cpp

@@ -2052,8 +2052,13 @@ Error Mle::SendChildUpdateRequest(ChildUpdateRequestMode aMode)
         ExitNow();
     }
 
-    mChildUpdateRequestState = kChildUpdateRequestActive;
-    ScheduleMessageTransmissionTimer();
+    if (aMode != kAppendZeroTimeout)
+    {
+        // Enable MLE retransmissions on all Child Update Request
+        // messages, except when actively detaching.
+        mChildUpdateRequestState = kChildUpdateRequestActive;
+        ScheduleMessageTransmissionTimer();
+    }
 
     VerifyOrExit((message = NewMleMessage(kCommandChildUpdateRequest)) != nullptr, error = kErrorNoBufs);
     SuccessOrExit(error = message->AppendModeTlv(mDeviceMode));