fix: reject zero ephemeral keypairs

[AztecBot]

Summary

Ephemeral keypair generation accepted a zero secret, which produces the point at infinity while still satisfying the positive-y check used by constrained note encryption. Rejecting infinity immediately after the fixed-base multiplication prevents constrained note delivery from emitting an undecryptable ephemeral public key.

Fix

• Reject point-at-infinity ephemeral public keys in both ephemeral keypair helpers.
• Add regressions that mock the random oracle to return zero and expect both helpers to fail.

Testing

• ../../noir/noir-repo/target/debug/nargo test --package noir_aztec --exact keys::ephemeral::test::generate_positive_ephemeral_key_pair_rejects_zero_secret --silence-warnings --test-threads 1 failed before the fix with Test passed when it should have failed
• ../../noir/noir-repo/target/debug/nargo test --package noir_aztec --exact keys::ephemeral::test::generate_positive_ephemeral_key_pair_rejects_zero_secret --silence-warnings --test-threads 1
• ../../noir/noir-repo/target/debug/nargo test --package noir_aztec --exact keys::ephemeral::test::generate_ephemeral_key_pair_rejects_zero_secret --silence-warnings --test-threads 1
• ../../noir/noir-repo/target/debug/nargo fmt --package noir_aztec
• ../../noir/noir-repo/target/debug/nargo test --package noir_aztec --no-run --silence-warnings
• git diff --check

Issues

• Closes AztecProtocol/aztec-claude#1048 — generate_positive_ephemeral_key_pair accepts zero secrets

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Created by claudebox · group: kernels-audit-remediation