crypto: cleanse AEAD Poly1305 key block #35476

This cleanses the temporary ChaCha20 block used to derive the one-time Poly1305 key in AEADChaCha20Poly1305::ComputeTag()

Poly1305 copies the first 32 bytes into its own context during construction, so the full temporary block can be wiped before processing AAD and ciphertext. This matches the existing cleanup of the FSChaCha20Poly1305 rekey block in the same file

No behavior change is expected for callers

Tested:

• cmake --build build --target test_bitcoin
• build/bin/test_bitcoin --run_test=crypto_tests
• ctest --test-dir build -R "^(crypto_tests|bip324_tests)$" --output-on-failure
• build/bin/test_bitcoin

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35476.

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Closing as suspected AI slop. If there is a human behind the PR that understands the code changes and can explain why they submitted them for review and how they reviewed themselves we may re open the PR.

Well, im a real contributor behind the account and i should have made the review context clearer before opening it maybe

The reason i submitted it is that ComputeTag() derives the one-time Poly1305 key from ChaCha20 block 0, then leaves the full temporary block on the stack after Poly1305 has copied the first 32 bytes into its context. Wiping that block after construction looked consistent with the existing memory_cleanse(one_block, sizeof(one_block)) in FSChaCha20Poly1305::NextPacket()

I also checked that this does not change ciphertext/tag behavior, and ran the tests i've written. 100% fine with the PR staying closed if this is considered too small or not worth, and i'll make something better next time :)

Edit: Tbf the change is pretty marginal and probably not worth reviewer time. Will do something better next time. Thanks!