fix: three Node.js compat bugs in node:crypto and node:buffer

[Sertug17]

Fixes three Node.js compatibility bugs found during manual code review.

Changes

1. node:crypto — Cipheriv.prototype.update default encoding

Fixes#6920

cipher.update('string') without inputEncoding now defaults to 'utf8' instead of throwing ERR_INVALID_ARG_VALUE, matching Node.js behavior.

2. node:crypto — Hmac.prototype.digest finalization

Fixes#6921

Calling hmac.digest() a second time now throws ERR_CRYPTO_HASH_FINALIZED instead of silently returning an empty Buffer, matching Hash.prototype.digest and Node.js behavior.

3. node:buffer — Buffer.alloc / allocUnsafe off-by-one

Fixes#6922

Changed size >= kMaxLength to size > kMaxLength so that Buffer.alloc(kMaxLength) no longer incorrectly throws ERR_OUT_OF_RANGE. The internal createBuffer function already used > — this makes alloc consistent.