[nat64] ensure to extend mapping lifetime on reuse (#11866)

Previously, a NAT64 mapping's lifetime was only extended upon its
initial allocation. This meant that if an existing mapping was found
and reused, its expiration timer would not be refreshed, potentially
leading to its premature removal.

This change moves the `mapping->Touch()` call from `AllocateMapping()` to
`TranslateFromIp6()`. This ensures that a mapping's lifetime is
extended every time it is used for translation, whether it is newly
allocated or an existing one being reused.

src/core/net/nat64_translator.cpp

@@ -163,6 +163,8 @@ Translator::Result Translator::TranslateFromIp6(Message &aMessage)
         ExitNow(result = kDrop);
     }
 
+    mapping->Touch(ip6Headers.GetIpProto());
+
 #if OPENTHREAD_CONFIG_NAT64_PORT_TRANSLATION_ENABLE
     srcPortOrId = mapping->mTranslatedPortOrId;
 #else
@@ -502,7 +504,6 @@ Translator::Mapping *Translator::AllocateMapping(const Ip6::Headers &aIp6Headers
     mapping->mSrcPortOrId        = GetSourcePortOrIcmp6Id(aIp6Headers);
     mapping->mTranslatedPortOrId = AllocateSourcePort(mapping->mSrcPortOrId);
 #endif
-    mapping->Touch(aIp6Headers.GetIpProto());
 
     LogInfo("Mapping created: %s", mapping->ToString().AsCString());