fix: return DATETIME columns as raw strings, default connection TZ to UTC

[devakone]

Summary

• Return MySQL DATETIME/DATE/TIMESTAMP values as raw strings with mysql2 dateStrings enabled.
• Default mysql2 connection timezone to UTC via MYSQL_TIMEZONE=Z, while allowing MYSQL_TIMEZONE overrides.
• Add unit coverage for pool options and a skipped-by-default MySQL integration test for DATETIME string round trips.
• Document the caller-visible Date-to-string behavior change. Versioning and changelog generation are left to release-please.

Validation

• npm test
• npm run build

Notes

• The real MySQL datetime integration test runs only when MYSQL_DATETIME_INTEGRATION=true and test database env vars are provided.
• This PR intentionally does not modify package.json version, CHANGELOG.md, package-lock.json, or .release-please-manifest.json.