Verify post-order traversal in test_pairs_in_post_order

[Copilot]

The test_pairs_in_post_order test verified the count of pairs but not their actual ordering in post-order traversal (children before parents).

Changes

• Added explicit assertions verifying inner pair (B . C) appears at index 0
• Added explicit assertions verifying outer pair (A . (B . C)) appears at index 1
• Added content validation for both pairs to confirm correct structure
• Added assertion confirming outer pair's right child references inner pair

// Verify that inner_pair is actually the (B . C) pair
match tree.allocator.sexp(inner_pair) {
    SExp::Pair(left, right) => {
        assert_eq!(tree.allocator.atom(left).as_ref(), &[2]);
        assert_eq!(tree.allocator.atom(right).as_ref(), &[3]);
    }
    _ => panic!("Expected inner_pair to be a pair"),
}

// Verify that outer_pair is actually the (A . (B . C)) pair
match tree.allocator.sexp(outer_pair) {
    SExp::Pair(left, right) => {
        assert_eq!(tree.allocator.atom(left).as_ref(), &[1]);
        assert_eq!(right, inner_pair, "Outer pair's right child should be the inner pair");
    }
    _ => panic!("Expected outer_pair to be a pair"),
}

This ensures the documented post-order behavior of InternedTree::pairs is properly tested.

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