Clarify 'explored nodes' vs 'queued nodes' terminology in maxNodes error message

Description

In include/na/zoned/layout_synthesizer/placer/HeuristicPlacer.hpp, the aStarTreeSearch function checks the open set size against maxNodes:

if (openSet.size() >= maxNodes) {
  throw std::runtime_error(
      "Maximum number of nodes reached. Increase max_nodes or increase "
      "deepening_value and deepening_factor to reduce the number of "
      "explored nodes.");
}

The guard is checking queued nodes (open set size), not expanded/explored nodes. The error message should be clarified to accurately reflect what is being limited.

Suggested fix

Either:

• Update the error message to say "queued nodes" instead of "explored nodes"
• Or track an explicit expanded counter if limiting explored nodes is the actual intent

Context

Identified during review of PR #875: https://github.com/munich-quantum-toolkit/qmap/pull/875
Comment: https://github.com/munich-quantum-toolkit/qmap/pull/875#discussion_r2616845968

cc: @ystade