bug(connections): URL parser treats sslmode=prefer as valid, but driver rejects it at connect time

[ZhuchkaTriplesix]

The connection URL parser in lib/features/connections/connection_url_parser.dart sets useSSL = true when sslmode is prefer, but the postgres package throws ArgumentError for sslmode=prefer at connect time.

Impact: A user can save a postgresql://...?sslmode=prefer URL via the UI and only discover it fails when they actually try to connect.

Expected: Reject prefer (or any unsupported sslmode value) at parse time, or at minimum do not mark it as SSL-enabled.

Related: test/core/database/postgres_connection_string_parse_test.dart already documents the driver behavior.

Acceptance criteria:

• parseConnectionUrlInput returns an error for sslmode=prefer on PostgreSQL URLs.
• Add unit tests covering prefer, allow, and other unsupported modes.