test: add regression tests for HMAC length-guard edge cases

Address review feedback on #1725:
- validateHMAC (item-based): same base64-length / different decoded-byte-length signature returns false without throwing
- validateBankingHMAC: same edge case
- secureCompare !received branch via empty/undefined signature
- fix missing trailing newline in hmacValidator.spec.ts

The Gemini early-return nit was already applied in the prior commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyuFNWN45FNffpGwsC4Su7