refactor(database): extract duplicate SSH tunnel lifecycle into TunnelableConnectionMixin #1508

Summary

Across all database connection implementations:

• lib/core/database/postgres_connection.dart (lines 121–348)
• lib/core/database/mysql_connection.dart (lines 126–338)
• lib/core/database/mongodb_connection.dart (lines 47–291)
• lib/core/database/redis_connection.dart

The exact same 40–50 lines of SSH tunneling logic are copy-pasted:

1. Field definitions:

   final SshTunnelConfig? sshConfig;
   final SshTunnelSecrets? sshSecrets;
   SshTunnelHandle? _sshTunnelHandle;
   SshTunnelHandle? get sshTunnelHandle => _sshTunnelHandle;

2. Opening tunnels:

   _sshTunnelHandle = await SshTunnelManager.instance.openTunnel(
     config: sshConfig!,
     secrets: sec ?? SshTunnelSecrets(),
   );

3. Host/port overriding (_sshTunnelHandle?.localHost, _sshTunnelHandle?.localPort).
4. Releasing tunnels in disconnect() and onDispose():

   await _sshTunnelHandle?.release();
   _sshTunnelHandle = null;

This violates DRY and increases the maintenance burden whenever tunnel timeouts, retries, or socket forwarding logic are improved.

Proposed Solution

1. Create TunnelableConnectionMixin in lib/core/database/tunnelable_connection_mixin.dart:
   • Manages SshTunnelHandle? _sshTunnelHandle.
   • Provides Future<({String host, int port})> openTunnelEndpoint(...).
   • Provides Future<void> releaseTunnel().
2. Mix TunnelableConnectionMixin into PostgresConnection, MySqlConnection, MongoConnection, and RedisConnection.
3. Standardize tunnel release semantics across all database drivers.

Acceptance Criteria

• Common SSH tunneling logic is centralized in a single mixin.
• No regression in connecting to PostgreSQL, MySQL, MongoDB, or Redis via SSH tunnels.
• Over 150 lines of duplicate boilerplate are removed across the database drivers.