validate backup size in BlackboardRestore

[aysha-afrah26]

BlackboardBackup and BlackboardRestore are meant to be used as a pair, but the restore side drives its loop with tree.subtrees.size() while indexing the backup vector the caller passed in. The only thing tying those two sizes together is an assert, and since CMakeLists.txt forces a Release build when no build type is given, NDEBUG is set in the default and ROS builds and that check is gone. Both sizes are XML-determined, since a tree contributes one blackboard per subtree plus one for the root, so restoring into a tree that was rebuilt from different XML than the backup came from walks off the end of the vector: ASan reports a heap-buffer-overflow read at bt_factory.cpp:779, and cloneInto is then called through the stale shared_ptr that read produced. I ran into it while looking at the rewind pattern from t17_blackboard_backup, where the backup and the tree are not necessarily created together. ImportTreeFromJSON right below already validates the same relationship and throws, so I made BlackboardRestore do the same instead of leaving the guard to a caller. The new test builds a one-subtree backup, restores it into a four-subtree tree, and checks that the matching case still works.