When a client detects subscription drop, mark sessions defunct. (#21841)

This marks defunct the session the subscripton is on, as well as any sessions
that are "more stale" than it, in the sense of not having seen anything from the
peer for at least as long.  This should increase the chance that our next
attempt to communicate with the peer will not use a stale session.

src/app/ReadClient.cpp

@@ -828,6 +828,29 @@ void ReadClient::OnLivenessTimeoutCallback(System::Layer * apSystemLayer, void *
                  "Subscription Liveness timeout with SubscriptionID = 0x%08" PRIx32 ", Peer = %02x:" ChipLogFormatX64,
                  _this->mSubscriptionId, _this->GetFabricIndex(), ChipLogValueX64(_this->GetPeerNodeId()));
 
+    // We didn't get a message from the server on time; it's possible that it no
+    // longer has a useful CASE session to us.  Mark defunct all sessions that
+    // have not seen peer activity in at least as long as our session.
+    const auto & holder = _this->mReadPrepareParams.mSessionHolder;
+    if (holder)
+    {
+        System::Clock::Timestamp lastPeerActivity = holder->AsSecureSession()->GetLastPeerActivityTime();
+        _this->mpImEngine->GetExchangeManager()->GetSessionManager()->ForEachMatchingSession(
+            _this->mPeer, [&lastPeerActivity](auto * session) {
+                if (!session->IsCASESession())
+                {
+                    return;
+                }
+
+                if (session->GetLastPeerActivityTime() > lastPeerActivity)
+                {
+                    return;
+                }
+
+                session->MarkAsDefunct();
+            });
+    }
+
     // TODO: add a more specific error here for liveness timeout failure to distinguish between other classes of timeouts (i.e
     // response timeouts).
     _this->Close(CHIP_ERROR_TIMEOUT);