Adds unit tests for the duplicate-database-name check (create + update
paths, plus the bypass-engine path), the SSH tunnel feature-flag and
db-port guards, and the SSH tunnel field-level error collection
(missing required fields, missing credentials, private key without
password). Brings patch coverage on commands/database/validate.py up
from ~44%.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>