make stable by default everywhere

Author: TrevorBergeron

bigframes/core/nodes.py

@@ -992,7 +992,7 @@ def remap_refs(
 @dataclasses.dataclass(frozen=True, eq=False)
 class OrderByNode(UnaryNode):
     by: Tuple[OrderingExpression, ...]
-    stable: bool = False
+    stable: bool = True
     # This is an optimization, if true, can discard previous orderings, even if doing a stable sort
     # might be a total ordering even if false
     is_total_order: bool = False