[mle] simplify SendChildUpdateRequest() (#8951)

This commit updates `Mle::SendChildUpdateRequest()`:
- It removes the use of default parameter.
- It adds a private version which accepts `ChildUpdateRequestMode`
  indicating actions like whether to append challenge TLV, or
  whether to append zero timeout value (used for graceful detach).

src/core/thread/mle.cpp

@@ -2027,14 +2027,14 @@ exit:
     return;
 }
 
-Error Mle::SendChildUpdateRequest(bool aAppendChallenge) { return SendChildUpdateRequest(aAppendChallenge, mTimeout); }
+Error Mle::SendChildUpdateRequest(void) { return SendChildUpdateRequest(kNormalChildUpdateRequest); }
 
-Error Mle::SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout)
+Error Mle::SendChildUpdateRequest(ChildUpdateRequestMode aMode)
 {
     Error                   error = kErrorNone;
     Ip6::Address            destination;
-    TxMessage              *message = nullptr;
-    AddressRegistrationMode mode    = kAppendAllAddresses;
+    TxMessage              *message     = nullptr;
+    AddressRegistrationMode addrRegMode = kAppendAllAddresses;
 
     if (!mParent.IsStateValidOrRestoring())
     {
@@ -2049,7 +2049,7 @@ Error Mle::SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout)
     VerifyOrExit((message = NewMleMessage(kCommandChildUpdateRequest)) != nullptr, error = kErrorNoBufs);
     SuccessOrExit(error = message->AppendModeTlv(mDeviceMode));
 
-    if (aAppendChallenge || IsDetached())
+    if ((aMode == kAppendChallengeTlv) || IsDetached())
     {
         mParentRequestChallenge.GenerateRandom();
         SuccessOrExit(error = message->AppendChallengeTlv(mParentRequestChallenge));
@@ -2058,13 +2058,13 @@ Error Mle::SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout)
     switch (mRole)
     {
     case kRoleDetached:
-        mode = kAppendMeshLocalOnly;
+        addrRegMode = kAppendMeshLocalOnly;
         break;
 
     case kRoleChild:
         SuccessOrExit(error = message->AppendSourceAddressTlv());
         SuccessOrExit(error = message->AppendLeaderDataTlv());
-        SuccessOrExit(error = message->AppendTimeoutTlv(aTimeout));
+        SuccessOrExit(error = message->AppendTimeoutTlv((aMode == kAppendZeroTimeout) ? 0 : mTimeout));
         SuccessOrExit(error = message->AppendSupervisionIntervalTlv(Get<SupervisionListener>().GetInterval()));
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
         if (Get<Mac::Mac>().IsCslEnabled())
@@ -2083,7 +2083,7 @@ Error Mle::SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout)
 
     if (!IsFullThreadDevice())
     {
-        SuccessOrExit(error = message->AppendAddressRegistrationTlv(mode));
+        SuccessOrExit(error = message->AppendAddressRegistrationTlv(addrRegMode));
     }
 
     destination.SetToLinkLocalAddress(mParent.GetExtAddress());
@@ -2707,7 +2707,7 @@ void Mle::ReestablishLinkWithNeighbor(Neighbor &aNeighbor)
 
     if (IsChild() && (&aNeighbor == &mParent))
     {
-        IgnoreError(SendChildUpdateRequest(/* aAppendChallenge */ true));
+        IgnoreError(SendChildUpdateRequest(kAppendChallengeTlv));
         ExitNow();
     }
 
@@ -4330,7 +4330,7 @@ Error Mle::DetachGracefully(otDetachGracefullyCallback aCallback, void *aContext
         break;
 
     case kRoleChild:
-        IgnoreError(SendChildUpdateRequest(/* aAppendChallenge */ false, /* aTimeout */ 0));
+        IgnoreError(SendChildUpdateRequest(kAppendZeroTimeout));
         break;
 
     case kRoleDisabled:

src/core/thread/mle.hpp

@@ -1565,13 +1565,11 @@ protected:
     /**
      * This method generates an MLE Child Update Request message.
      *
-     * @param[in] aAppendChallenge   Indicates whether or not to include a Challenge TLV (even when already attached).
-     *
      * @retval kErrorNone    Successfully generated an MLE Child Update Request message.
      * @retval kErrorNoBufs  Insufficient buffers to generate the MLE Child Update Request message.
      *
      */
-    Error SendChildUpdateRequest(bool aAppendChallenge = false);
+    Error SendChildUpdateRequest(void);
 
     /**
      * This method generates an MLE Child Update Response message.
@@ -1846,6 +1844,13 @@ private:
         kChildUpdateRequestActive,  // Child Update Request has been sent and Child Update Response is expected.
     };
 
+    enum ChildUpdateRequestMode : uint8_t // Used in `SendChildUpdateRequest()`
+    {
+        kNormalChildUpdateRequest, // Normal Child Update Request.
+        kAppendChallengeTlv,       // Append Challenge TLV to Child Update Request even if currently attached.
+        kAppendZeroTimeout,        // Use zero timeout when appending Timeout TLV (used for graceful detach).
+    };
+
     enum DataRequestState : uint8_t
     {
         kDataRequestNone,   // Not waiting for a Data Response.
@@ -1978,7 +1983,7 @@ private:
     static void HandleDetachGracefullyTimer(Timer &aTimer);
     void        HandleDetachGracefullyTimer(void);
     bool        IsDetachingGracefully(void) { return mDetachGracefullyTimer.IsRunning(); }
-    Error       SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout);
+    Error       SendChildUpdateRequest(ChildUpdateRequestMode aMode);
 
 #if OPENTHREAD_CONFIG_MLE_LINK_METRICS_INITIATOR_ENABLE || OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE
     Error SendDataRequest(const Ip6::Address                        &aDestination,