fix: reject invalid attachment digest lengths

[Ap4sh]

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Description

This rejects attachment digests unless their length matches the SHA-256 digest computed for the encrypted attachment

verifyDigest() previously compared only theirDigest.byteLength bytes, so a truncated digest could be accepted after the MAC check passed

The MAC check still protects the encrypted payload first, so this is a small correctness hardening fix, not a ciphertext behavior change

Added a unit test covering a valid encrypt/decrypt round trip and a truncated digest rejection

Tests:

• pnpm ready
• npx -y node@24.12.0 node_modules/mocha/bin/mocha.js --no-config --timeout 10000 app/ts/test/session/unit/crypto/attachmentsEncrypter_test.js