[mle] introduce Attacher class (#11835)

This commit introduces a new `Mle::Attacher` class to encapsulate all
logic and state related to the device attach process.

This change moves the attach state machine, parent candidate
management, attach timer, and message handling for Parent Request,
Parent Response, Child ID Request, and Child ID Response from `Mle`
into the new nested `Attacher` class.

This refactoring improves code organization and modularity by
separating the attach logic from the main `Mle` class.

src/core/thread/mle_ftd.cpp

@@ -305,7 +305,7 @@ void Mle::HandleDetachStart(void)
     Get<TimeTicker>().UnregisterReceiver(TimeTicker::kMle);
 }
 
-void Mle::HandleChildStart(AttachMode aMode)
+void Mle::HandleChildStart(void)
 {
     mAddressSolicitRejected = false;
 
@@ -323,7 +323,7 @@ void Mle::HandleChildStart(AttachMode aMode)
 
     VerifyOrExit(IsRouterIdValid(mPreviousRouterId));
 
-    switch (aMode)
+    switch (mAttacher.GetAttachMode())
     {
     case kDowngradeToReed:
         SendAddressRelease();
@@ -411,10 +411,8 @@ void Mle::SetStateRouterOrLeader(DeviceRole aRole, uint16_t aRloc16, LeaderStart
     SetRole(aRole);
 
     mPrevRoleRestorer.Stop();
+    mAttacher.CancelAttachOnRoleChange();
 
-    SetAttachState(kAttachStateIdle);
-    mAttachCounter = 0;
-    mAttachTimer.Stop();
     mRetxTracker.Stop();
     StopAdvertiseTrickleTimer();
     ResetAdvertiseInterval();
@@ -1213,7 +1211,7 @@ Error Mle::HandleAdvertisementOnFtd(RxInfo &aRxInfo, uint16_t aSourceAddress, co
 #endif
         )
         {
-            Attach(kBetterPartition);
+            mAttacher.Attach(kBetterPartition);
         }
 
         ExitNow(error = kErrorDrop);
@@ -1590,13 +1588,13 @@ void Mle::HandleTimeTick(void)
         if ((mRouterTable.GetActiveRouterCount() > 0) && (mRouterTable.GetLeaderAge() >= mNetworkIdTimeout))
         {
             LogInfo("Leader age timeout");
-            Attach(kSamePartition);
+            mAttacher.Attach(kSamePartition);
         }
 
         if (roleTransitionTimeoutExpired && mRouterTable.GetActiveRouterCount() > mRouterDowngradeThreshold)
         {
             LogNote("Downgrade to REED");
-            Attach(kDowngradeToReed);
+            mAttacher.Attach(kDowngradeToReed);
         }
 
         OT_FALL_THROUGH;
@@ -3220,7 +3218,7 @@ void Mle::RemoveNeighbor(Neighbor &aNeighbor)
     }
     else if (&aNeighbor == &GetParentCandidate())
     {
-        ClearParentCandidate();
+        mAttacher.ClearParentCandidate();
     }
     else if (IsChildRloc16(aNeighbor.GetRloc16()))
     {