Reject oversized VK metadata integers

[AztecBot]

Summary

Verification-key metadata deserialization accepted field and uint256_t words wider than the target integer type, then truncated them through static_cast. This change rejects out-of-range uint32_t and uint64_t encodings in both native VK codec paths before casting, so malformed metadata cannot round-trip as a different value.

Fix

• Add bounded integer deserialization for uint32_t and uint64_t in FrCodec and U256Codec.
• Add codec regressions for 2^32 + x and 2^64 + x, plus max-value acceptance checks.
• Add a VK from_field_elements regression covering oversized metadata across Ultra, Mega, and UltraKeccak flavor parameters.

Testing

• cmake --build --preset default --target ecc_tests -j$(nproc)
• ctest --preset default -R 'FieldConversionTest\.(FrCodecRejectsOversizedUint32Metadata|FrCodecRejectsOversizedUint64Metadata|U256CodecRejectsOversizedUint32Metadata|U256CodecRejectsOversizedUint64Metadata|IntegerMetadataMaxValuesAccepted|FieldConversionUint32)$' --output-on-failure
• cmake --build --preset default --target flavor_tests -j$(nproc)
• ctest --preset default -R 'FromFieldElementsRejectsOversizedMetadata' --output-on-failure

Issues

• Closes AztecProtocol/barretenberg-claude#2516 — reject oversized VK metadata integer encodings before truncating casts.
• Closes AztecProtocol/barretenberg-claude#3348 — apply the same non-canonical integer policy to the companion VK helper surface.

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Created by claudebox · group: bb-audit-remediation