[metadata] update return type of ParentNodeProvider to be CSTNode

[jimmylai]

Summary

@Kronuz called out that the return type of ParentNodeProvider should NOT be an Optional.

That's true because the original_node set as metadata is always a CSTNode.

LibCST/libcst/metadata/parent_node_provider.py

Lines 18 to 21 in c023fa7

	def on_leave(self, original_node: cst.CSTNode) -> None:
	for child in original_node.children:
	self.provider.set_metadata(child, original_node)
	super().on_leave(original_node)

Test Plan

Existing tests.

[Kronuz]