Fix/xwing integrity length check

[sverma98015-commits]

Enforce correct byte size check and prevent out-of-bounds slicing in X-Wing integrityCheckedRepresentation initializer.

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

Motivation

The XWingMLKEM768X25519.PrivateKey initializer for integrityCheckedRepresentation previously enforced a guard check requiring a total count of 96 bytes. However, the X-Wing private key structure under the hood expects a 32-byte seed representation concatenated with a 32-byte SHA3-256 public key hash suffix, making the actual payload size exactly 64 bytes.

Because of this size mismatch, passing a valid 64-byte representation threw an incorrectParameterSize error. Additionally, attempting to slice the trailing 32 bytes using .dropFirst(32) on a strictly 64-byte collection was prone to slicing index issues depending on the underlying DataProtocol backing type.

Modifications

• Updated the size verification guard check in init(integrityCheckedRepresentation:) from 96 to strictly enforce 64 bytes.
• Swapped out the starting-slice logic for publicKeyHashBytes to use .suffix(32), ensuring a reliable slice of the final 32 bytes regardless of the input data layout.

Result

• Valid X-Wing integrityCheckedRepresentation keys can now be successfully initialized without triggering a parameter size mismatch error.
• All 4 targeted validation unit tests in XWingTests now pass successfully.