Fix test compatibility: Restore backward-compatible error message format

The improved error messages from Phase 4 changed the format from:
  'Given index is not found.'
To:
  'Cannot erase index X: not found in tree (tree size: Y)'

Tests expected the old format substring 'Given index is not found'.

SOLUTION:
Restore backward-compatible format while keeping Phase 4 improvements:
  'Given index is not found. (Index: X, tree size: Y)'

This maintains:
✓ Test compatibility (contains expected substring)
✓ Phase 4 improvements (contextual information)
✓ Better debugging (specific index and tree size shown)

All 12 erase error tests now pass.

Author: claude

cpp/prtree.h

@@ -1430,10 +1430,10 @@ template <IndexType T, int B = 6, int D = 2> class PRTree {
 
     auto it = idx2bb.find(idx);
     if (unlikely(it == idx2bb.end())) {
-      // Phase 4: Improved error message with context
+      // Phase 4: Improved error message with context (backward compatible)
       throw std::runtime_error(
-          "Cannot erase index " + std::to_string(idx) +
-          ": not found in tree (tree size: " + std::to_string(idx2bb.size()) + ")");
+          "Given index is not found. (Index: " + std::to_string(idx) +
+          ", tree size: " + std::to_string(idx2bb.size()) + ")");
     }
     BB<D> target = it->second;