test: cover checkpoint boundary variants

[AztecBot]

Summary

The checkpoint/root variant tests missed several legal boundary cases around padding and single-child shapes. This adds committed coverage for the padding circuit output, root right-padding with a general checkpoint-root child, and all allowed first-block VKs in the checkpoint-root single-block path.

Fix

• Assert the checkpoint padding rollup returns empty checkpoint public inputs.
• Add root right-padding success coverage for a general one-checkpoint root child.
• Expand checkpoint-root single-block success coverage across all allowed first-block VK indices.

Testing

• PATH=/home/aztec-dev/aztec-packages/noir/noir-repo/target/debug:$PATH nargo test --package rollup_lib checkpoint_padding_is_empty with_left_general_root_and_right_padding with_single_block_root --silence-warnings

Issues

• Closes AztecProtocol/aztec-claude#1628 — checkpoint padding and single-child variant tests miss legal boundary cases

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Created by claudebox · group: rollup-audit-remediation