fix(extensions): edit-connection UI parity (#518)

[ZhuchkaTriplesix]

Summary

• SDUI edit mode: blank required passwords validate; hint Leave blank to keep existing (keepExistingSecrets)
• Extension Test Connection merges stored secrets like Save
• Extension sidebar: remove inline × — Remove only via context menu (matches built-ins)
• Extract connection_edit_secrets.dart so extension form can merge without a circular import

Closes #518

Test plan

• Edit extension connection with required password → leave blank → Save keeps secret
• Same edit → Test Connection with blank password uses stored secret
• Extension tile has no trailing ×; Remove only from context menu
• Create extension connection still requires password when schema marks it required
• flutter test test/core/sdui/sdui_builders_test.dart test/features/connections/connection_edit_helpers_test.dart test/core/storage/local_db_secrets_test.dart