[routing-manager] add Publish()/Unpublish() in Nat64PrefixManager (#11838)

This commit introduces `Publish()` and `Unpublish()` private helper
methods in `Nat64PrefixManager` to encapsulate and centralize the
logic for managing the published NAT64 prefix in the Network Data.

- `Publish(aPrefix, aPreference)` adds the given prefix to the Network
  Data. It handles removing any previously published prefix if the
  new prefix or preference differs.
- `Unpublish()` removes the currently published NAT64 prefix from the
  Network Data, if one exists.

These new helpers simplify the logic within the `Evaluate()` and
`Stop()` methods, reduce code duplication, and improve overall
clarity.

src/core/border_router/routing_manager.cpp

@@ -4055,12 +4055,7 @@ void RoutingManager::Nat64PrefixManager::Stop(void)
 {
     LogInfo("Stopping Nat64PrefixManager");
 
-    if (mPublishedPrefix.IsValidNat64())
-    {
-        IgnoreError(Get<NetworkData::Publisher>().UnpublishPrefix(mPublishedPrefix));
-    }
-
-    mPublishedPrefix.Clear();
+    Unpublish();
     mInfraIfPrefix.Clear();
     mTimer.Stop();
 
@@ -4129,17 +4124,13 @@ void RoutingManager::Nat64PrefixManager::Evaluate(void)
         ((error == kErrorNotFound) || (netdataPrefixConfig.mPreference < preference) ||
          (netdataPrefixConfig.GetPrefix() == mPublishedPrefix) || (netdataPrefixConfig.GetPrefix() == mInfraIfPrefix));
 
-    if (mPublishedPrefix.IsValidNat64() && (!shouldPublish || (prefix != mPublishedPrefix)))
+    if (shouldPublish)
     {
-        IgnoreError(Get<NetworkData::Publisher>().UnpublishPrefix(mPublishedPrefix));
-        mPublishedPrefix.Clear();
+        Publish(prefix, preference);
     }
-
-    if (shouldPublish && ((prefix != mPublishedPrefix) || (preference != mPublishedPreference)))
+    else
     {
-        mPublishedPrefix     = prefix;
-        mPublishedPreference = preference;
-        Publish();
+        Unpublish();
     }
 
 #if OPENTHREAD_CONFIG_NAT64_TRANSLATOR_ENABLE
@@ -4162,10 +4153,25 @@ exit:
     return;
 }
 
-void RoutingManager::Nat64PrefixManager::Publish(void)
+void RoutingManager::Nat64PrefixManager::Publish(const Ip6::Prefix &aPrefix, RoutePreference aPreference)
 {
+    // Publishes the given prefix and preference in the Network Data
+    // if they differ from the currently published ones. This method
+    // updates `mPublishedPrefix` and `mPublishedPreference` to track
+    // the new values.
+    //
+    // If there is any change, this method ensures that the previous
+    // prefix is unpublished before the new one is published.
+
     NetworkData::ExternalRouteConfig routeConfig;
 
+    VerifyOrExit((aPrefix != mPublishedPrefix) || (aPreference != mPublishedPreference));
+
+    Unpublish();
+
+    mPublishedPrefix     = aPrefix;
+    mPublishedPreference = aPreference;
+
     routeConfig.Clear();
     routeConfig.SetPrefix(mPublishedPrefix);
     routeConfig.mPreference = mPublishedPreference;
@@ -4174,6 +4180,23 @@ void RoutingManager::Nat64PrefixManager::Publish(void)
 
     SuccessOrAssert(
         Get<NetworkData::Publisher>().PublishExternalRoute(routeConfig, NetworkData::Publisher::kFromRoutingManager));
+
+exit:
+    return;
+}
+
+void RoutingManager::Nat64PrefixManager::Unpublish(void)
+{
+    // Unpublishes the previously published prefix (if any) and clears
+    // the `mPublishedPrefix`.
+
+    VerifyOrExit(mPublishedPrefix.IsValidNat64());
+
+    IgnoreError(Get<NetworkData::Publisher>().UnpublishPrefix(mPublishedPrefix));
+    mPublishedPrefix.Clear();
+
+exit:
+    return;
 }
 
 void RoutingManager::Nat64PrefixManager::HandleTimer(void)

src/core/border_router/routing_manager.hpp

@@ -1556,12 +1556,12 @@ private:
 
     private:
         void Discover(void);
-        void Publish(void);
+        void Publish(const Ip6::Prefix &aPrefix, RoutePreference aPreference);
+        void Unpublish(void);
 
         using Nat64Timer = TimerMilliIn<RoutingManager, &RoutingManager::HandleNat64PrefixManagerTimer>;
 
-        bool mEnabled;
-
+        bool            mEnabled;
         Ip6::Prefix     mInfraIfPrefix;       // The latest NAT64 prefix discovered on the infrastructure interface.
         Ip6::Prefix     mLocalPrefix;         // The local prefix (from BR ULA prefix).
         Ip6::Prefix     mPublishedPrefix;     // The prefix to publish in Net Data (empty or local or from infra-if).