bug(connections): form 'Test Connection' hides the real SSL failure reason

[ZhuchkaTriplesix]

In postgresql_connection_form.dart, _testConnection calls conn.testConnection() and maps the boolean result to either 'success' or 'failed'. The catch block only shows the error if connect() throws an exception before the boolean check.

Impact: When testConnection() returns false due to an SSL or auth problem, the user sees just "Connection failed" with no diagnosis.

Expected: Always show the real failure cause from testConnection.

Acceptance criteria:

• Propagate error message from testConnection to the banner.
• Update widget tests to assert that the specific error text is shown.