log,net: avoid ComputeTotalSize when logging is disabled

`PeerManagerImpl::SendBlockTransactions()` computed the total byte size of requested transactions for a debug log line by calling `ComputeTotalSize()` in a tight loop, triggering serialization even when debug logging is off.

Guard the size accumulation with `LogAcceptCategory` so the serialization work only happens when the log line can be emitted.

No behavior change when debug logging is enabled: the reported block hash, transaction count, and byte totals are the same.
The bounds checks still run unconditionally; the debug-only loop iterates the already-validated response contents.

Separating debug-only work from the critical path reduces risk and favors the performance-critical non-debug case.
This also narrows the racy scope of when logging is toggled from another thread.

src/net_processing.cpp

@@ -2570,17 +2570,19 @@ uint32_t PeerManagerImpl::GetFetchFlags(const Peer& peer) const
 void PeerManagerImpl::SendBlockTransactions(CNode& pfrom, Peer& peer, const CBlock& block, const BlockTransactionsRequest& req)
 {
     BlockTransactions resp(req);
-    unsigned int tx_requested_size = 0;
     for (size_t i = 0; i < req.indexes.size(); i++) {
         if (req.indexes[i] >= block.vtx.size()) {
             Misbehaving(peer, "getblocktxn with out-of-bounds tx indices");
             return;
         }
         resp.txn[i] = block.vtx[req.indexes[i]];
-        tx_requested_size += resp.txn[i]->ComputeTotalSize();
     }
 
-    LogDebug(BCLog::CMPCTBLOCK, "Peer %d sent us a GETBLOCKTXN for block %s, sending a BLOCKTXN with %u txns. (%u bytes)\n", pfrom.GetId(), block.GetHash().ToString(), resp.txn.size(), tx_requested_size);
+    if (LogAcceptCategory(BCLog::CMPCTBLOCK, BCLog::Level::Debug)) {
+        uint32_t tx_requested_size{0};
+        for (const auto& tx : resp.txn) tx_requested_size += tx->ComputeTotalSize();
+        LogDebug(BCLog::CMPCTBLOCK, "Peer %d sent us a GETBLOCKTXN for block %s, sending a BLOCKTXN with %u txns. (%u bytes)\n", pfrom.GetId(), block.GetHash().ToString(), resp.txn.size(), tx_requested_size);
+    }
     MakeAndPushMessage(pfrom, NetMsgType::BLOCKTXN, resp);
 }