Remove alphabetical sort in descend_tree for overlapping children

When multiple tree children match a structure, the previous fix sorted
them by label and picked the first. This breaks when siblings overlap
(e.g. Nitrites sorts before Nitro alphabetically, but Nitro is the
intended match because it appears first in the tree file).

The tree is constructed deterministically from the database files, so
the children list order is already deterministic. Simply picking the
first matching child (next_node[0]) restores correct behavior while
remaining deterministic.

Fixes:
- test_statmech: Nitro sample molecule was matching Nitrites (alphabetically
  first) instead of Nitro (tree order first)
- test_degeneracy_keeps_separate_transition_states_separated
- test_add_ring_correction_thermo_data_from_tree_for_existing_tricyclic

Author: JacksonBurns

rmgpy/data/base.py

@@ -1066,11 +1066,10 @@ def descend_tree(self, structure, atoms, root=None, strict=False):
             else:
                 return root
         else:
-            # Multiple children match - sort by node label for deterministic selection
-            next_node.sort(key=lambda n: n.label)
-            # logging.warning('For {0}, a node {1} with overlapping children {2} was encountered '
-            #                 'in tree with top level nodes {3}. Assuming the first match is the '
-            #                 'better one.'.format(structure, root, next, self.top))
+            # Multiple children match - pick the first in tree order.
+            # The tree is constructed deterministically, so this is deterministic.
+            # (Sorting by label can pick the wrong node when siblings overlap,
+            # e.g. Nitrites sorts before Nitro but Nitro is the intended match.)
             return self.descend_tree(structure, atoms, next_node[0], strict)
 
     def are_siblings(self, node, node_other):