fix(connections): Redis and MongoDB Test Connection says only "failed", without the reason #1308

Problem

RedisConnection.testConnection() (lib/core/database/redis_connection.dart) and MongoConnection.testConnection() return bool. The forms show 'success' or 'failed':

• lib/features/redis/redis_connection_form.dart:201
• lib/features/mongodb/mongodb_connection_form.dart:305

The mapped reason (Authentication failed, Cannot reach the database server, timed out, the SSH reason) is dropped (a debugPrint at most). PostgreSQL and MySQL already return (ok, error) and show the message.

Scope

• Return ({bool ok, String? error}) from both, with describeDatabaseError, as the PostgreSQL / MySQL tests do.
• Show the message in the forms.

Acceptance

• A wrong Redis password shows Authentication failed with its hint.
• An unreachable MongoDB host shows the unreachable message.
• A successful test still shows success.