fix(connections): Test Connection says why it failed (#1308)

[ZhuchkaTriplesix]

Closes #1308

Problem

• RedisConnection, MysqlConnection and MongoConnection.testConnection() returned bool; the forms showed failed and the mapped reason (Authentication failed, Cannot reach the database server, ...) was at most a debugPrint.
• Worse, every form (PostgreSQL, SQLite too) rendered 'Connection failed' for any result that did not start with error:, so even the reason PostgreSQL returned was never shown.
• The SQLite form's error: results were cut with substring(7) after a 6-character prefix: the first letter of the message was lost.

Fix

• The three methods return ({bool ok, String? error}), with describeDatabaseError (as the PostgreSQL / SQLite ones do).
• connection_test_result.dart: connectionTestMessage shows the reason (success / failed / error: ... / a reason), connectionTestResultLifetime keeps a reason on screen for 20 s (5 s for success). The five forms use them.

Tests: the message helper (incl. the SQLite prefix); RedisConnection.testConnection reports Authentication failed for a wrong password (tiny RESP server) and Cannot reach for a closed port; MysqlConnection.testConnection Cannot reach for a closed port. MongoDB is not driven in a unit test: the driver retries against a closed port for a long time.