[nexus] add mechanism to emulate node reset (#11662)

This commit adds a new mechanism to emulate a node reset on
`Nexus::Node`. This is realized by resetting all platform components
while ensuring the non-volatile `mSettings` remains unchanged, then
reinitializing the `ot::Instance` by invoking its constructor.

This is used to add a new `test_full_network_reset` test, which
emulates a full simultaneous reset of all nodes in a large network,
tracking how long it takes for the network to stabilize after the
reset event.

tests/nexus/platform/nexus_node.cpp

@@ -32,6 +32,26 @@
 namespace ot {
 namespace Nexus {
 
+void Node::Reset(void)
+{
+    Instance *instance = &GetInstance();
+    uint32_t  id       = GetId();
+
+    mRadio.Reset();
+    mAlarm.Reset();
+    mMdns.Reset();
+    mPendingTasklet = false;
+#if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE
+    mTrel.Reset();
+#endif
+
+    instance->~Instance();
+
+    instance = new (instance) Instance();
+    instance->SetId(id);
+    instance->AfterInit();
+}
+
 void Node::Form(void)
 {
     MeshCoP::Dataset::Info datasetInfo;