fix(mysql): forceClose during connect leaks the SSH tunnel handle, and the interrupted session is revived #1313

Problem

MysqlConnection.forceClose (lib/core/database/mysql_connection.dart:404) returns early with if (c == null) return;, before the SSH handle is released. When it runs while connect() is in flight (tunnel opened, _conn not yet assigned), the handle is never released and the tunnel session keeps a reference forever.

Also, _connectOnce sets _isConnected = true when it completes, so a connection that was interrupted in that window comes back as connected. PostgresConnection.forceClose releases the handle regardless.

Scope

• Release the tunnel handle before the early return, as PostgreSQL does.
• Mark the in-flight attempt as cancelled so it closes what it opened instead of reviving it.
• Test: forceClose while a (fake) connect is pending.

Acceptance

• The tunnel's ref-count is back to zero after forceClose during connect.
• The interrupted connection is not connected afterwards.