bug(connections): URL parser does not validate PostgreSQL sslmode values

[ZhuchkaTriplesix]

connection_url_parser.dart accepts any sslmode string. Values like prefer, allow, invalid are silently passed through to the driver and cause runtime crashes.

Impact: Bad UX: failures happen late, during connect, not at input time.

Expected: Validate and reject unsupported sslmode values while parsing the URL.

Acceptance criteria:

• Reject sslmode values not in {disable, require, verify-ca, verify-full} for PostgreSQL.
• Return a clear error message from parseConnectionUrlInput.