op_cache, proxy_client: honor GetCallback return and guard push refresh expiry

- SearchCache::get: if gcb returns false, call dcb(false, {}) instead
  of silently proceeding with dcb(true, {}).
- DhtProxyClient push refresh: skip expiration of old values when the
  get() completion reports failure (ok == false), preventing spurious
  expires on transient errors.

In pushNotificationReceived, track the set of value IDs received
in the refresh GET and only expire values whose IDs are not in the
new set.  Previously all old values were expired on refresh,
incorrectly expiring edited values.

src/dht_proxy_client.cpp

@@ -1310,13 +1310,25 @@ DhtProxyClient::pushNotificationReceived([[maybe_unused]] const std::map<std::st
                 if (expired == notification.end()) {
                     auto cb = list.second.cb;
                     auto oldValues = list.second.cache.getValues();
+                    auto receivedIds = std::make_shared<std::set<Value::Id>>();
                     get(
                         key,
-                        [cb, sendTime](const std::vector<Sp<Value>>& vals) { return cb(vals, false, sendTime); },
-                        [cb, oldValues, sendTime](bool /*ok*/) {
+                        [cb, sendTime, receivedIds](const std::vector<Sp<Value>>& vals) {
+                            for (const auto& v : vals)
+                                receivedIds->insert(v->id);
+                            return cb(vals, false, sendTime);
+                        },
+                        [cb, oldValues, sendTime, receivedIds](bool ok) {
+                            if (!ok)
+                                return;
                             // Decrement old values refcount to expire values not
                             // present in the new list
-                            cb(oldValues, true, sendTime);
+                            std::vector<Sp<Value>> toExpire;
+                            for (const auto& v : oldValues)
+                                if (receivedIds->find(v->id) == receivedIds->end())
+                                    toExpire.emplace_back(v);
+                            if (!toExpire.empty())
+                                cb(toExpire, true, sendTime);
                         });
                     ret = PushNotificationResult::Values;
                 } else {

src/op_cache.cpp

@@ -244,15 +244,17 @@ SearchCache::expire(const time_point& now, const std::function<void(size_t)>& on
 bool
 SearchCache::get(const Value::Filter& f, const Sp<Query>& q, const GetCallback& gcb, const DoneCallback& dcb) const
 {
+    if (not gcb)
+        return false;
     auto op = getOp(q);
-    if (op != ops.end() and op->second->isSynced()) {
-        auto vals = op->second->get(f);
-        if (not vals.empty())
-            gcb(vals);
+    if (op == ops.end() or not op->second->isSynced())
+        return false;
+    auto vals = op->second->get(f);
+    if (not vals.empty())
+        gcb(vals); // a consumer stopping early is not a failure
+    if (dcb)
         dcb(true, {});
-        return true;
-    }
-    return false;
+    return true;
 }
 
 std::vector<Sp<Value>>