fix: allow fs error codes in the DotenvError code union

config() returns whatever fs.readFileSync threw for a path it could not
read, so ENOENT reaches the caller while the declared union admits only
OBJECT_REQUIRED and SECURE_REQUIRES_DOTENVX.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9eeZobBsqv7R5kTkNUN5L