[nexus] fix segmentation fault during Core destruction (#12866)

When the Nexus test finishes, it automatically destructs all its allocated
Nodes sequentially. During this destruction phase, the OpenThread instance
attempts to destruct objects like `Nat64::Translator`, which might in turn
call logging mechanisms like `Mapping::Free()` that rely on the static
`Instance::GetActiveInstance()` pointer.

Because `Core::~Core()` did not maintain or update `gActiveInstance` while
iterating through node destructors, this pointer was left dangling, causing
segmentation faults when dereferenced by `ot::Instance::GetLogLevel()`.

This commit fixes `Core::~Core()` to manually loop through and destruct the
`mNodes` list, calling `UpdateActiveInstance(&node->GetInstance())` right
before destroying each node. This ensures that `gActiveInstance` points to
the correct context while node destruction logic runs.

tests/nexus/platform/nexus_core.cpp

@@ -331,8 +331,19 @@ void Core::AddOmrPrefixTestVar(const char *aName, Node &aNode)
     OT_UNUSED_VARIABLE(aNode);
 #endif
 }
+Core::~Core(void)
+{
+    while (!mNodes.IsEmpty())
+    {
+        Node *node = mNodes.GetHead();
 
-Core::~Core(void) { sInUse = false; }
+        UpdateActiveInstance(&node->GetInstance());
+        mNodes.Pop();
+    }
+
+    UpdateActiveInstance(nullptr);
+    sInUse = false;
+}
 
 Node &Core::CreateNode(void)
 {