bug(connections): PostgresConnection.testConnection swallows SSL and network errors

[ZhuchkaTriplesix]

PostgresConnection.testConnection() in lib/core/database/postgres_connection.dart catches all exceptions and returns false, discarding the actual error message.

Impact: Users see only the generic banner text "Connection failed" in the form, even when the real cause is a certificate rejection or SSL handshake failure.

Expected: Surface the underlying error so the UI can display actionable feedback.

Acceptance criteria:

• testConnection returns a result object {bool ok, String? error} or throws a typed exception.
• Form UI displays the real message for non-success results.
• Existing callers updated and tested.