[mle] use PrevRoleRestorer for child update challenge (#11801)

This commit updates the challenge/response mechanism used when a
detached device sends a "Child Update Request" to restore its role as
a child.

Previously, this process shared the `mParentRequestChallenge` with the
parent search mechanism. This logic is now consolidated within the
`PrevRoleRestorer` class, which now manages the generation and
tracking of the `TxChallenge` used in "Child Update Request".

This change simplifies the `Mle` class design and makes the child role
restoration logic separate from the parent search and attach process.
This separation allows for future enhancements where a device may run
both mechanisms in parallel.

src/core/thread/mle.cpp

@@ -1912,10 +1912,16 @@ Error Mle::SendChildUpdateRequestToParent(ChildUpdateRequestMode aMode)
     VerifyOrExit((message = NewMleMessage(kCommandChildUpdateRequest)) != nullptr, error = kErrorNoBufs);
     SuccessOrExit(error = message->AppendModeTlv(mDeviceMode));
 
-    if ((aMode == kAppendChallengeTlv) || IsDetached())
+    switch (aMode)
     {
-        mParentRequestChallenge.GenerateRandom();
-        SuccessOrExit(error = message->AppendChallengeTlv(mParentRequestChallenge));
+    case kNormalChildUpdateRequest:
+    case kAppendZeroTimeout:
+        break;
+    case kAppendChallengeTlv:
+    case kToRestoreChildRole:
+        mPrevRoleRestorer.GenerateRandomChallenge();
+        SuccessOrExit(error = message->AppendChallengeTlv(mPrevRoleRestorer.GetChallenge()));
+        break;
     }
 
     switch (mRole)
@@ -3425,7 +3431,8 @@ void Mle::HandleChildUpdateResponseOnChild(RxInfo &aRxInfo)
     switch (mRole)
     {
     case kRoleDetached:
-        VerifyOrExit(response == mParentRequestChallenge, error = kErrorSecurity);
+        VerifyOrExit(mPrevRoleRestorer.IsRestoringChildRole(), error = kErrorSecurity);
+        VerifyOrExit(response == mPrevRoleRestorer.GetChallenge(), error = kErrorSecurity);
         break;
 
     case kRoleChild:

src/core/thread/mle.hpp

@@ -1755,10 +1755,8 @@ private:
         bool  IsRestoringRouterOrLeaderRole(void) const { return mState == kRestoringRouterOrLeaderRole; }
         void  HandleTimer(void);
 
-#if OPENTHREAD_FTD
         void               GenerateRandomChallenge(void) { mChallenge.GenerateRandom(); }
         const TxChallenge &GetChallenge(void) const { return mChallenge; }
-#endif
 
     private:
         static constexpr uint32_t kMaxStartDelay                = 25;
@@ -1782,12 +1780,10 @@ private:
 
         using DelayTimer = TimerMilliIn<Mle, &Mle::HandleRoleRestorerTimer>;
 
-        State      mState;
-        uint8_t    mAttempts;
-        DelayTimer mTimer;
-#if OPENTHREAD_FTD
+        State       mState;
+        uint8_t     mAttempts;
+        DelayTimer  mTimer;
         TxChallenge mChallenge;
-#endif
     };
 
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