fix(redis): connect has no timeout, leaks the socket after a failed AUTH, and is not single-flight #1307

Problem

lib/core/database/redis_connection.dart (_openConnection, ~lines 142-210):

1. No timeout. _conn.connect(host, port) and SecureSocket.connect(...) have no timeout, unlike PostgreSQL / MySQL (10 s). A host that drops packets keeps the spinner and Test Connection busy for the OS TCP timeout (about two minutes).
2. Leaked socket. When AUTH or PING throws, the catch sets _conn = null and _command = null without closing the connection. disconnect() closes only if (c != null && wasConnected). Every failed attempt, including each Test Connection with a wrong password, leaves an open socket.
3. No single-flight. PostgreSQL and MySQL made connect() single-flight (two callers opened two sockets and two SSH tunnels). Redis still allows two concurrent _openConnections: the second overwrites _conn, the first socket and its tunnel handle are leaked.

Scope

• Add a connect timeout (10 s, configurable like the others) to plain and TLS connects.
• Close the connection in the failure path.
• Single-flight connect() as in PostgresConnection.connect.
• Tests with a fake server / injected connector: timeout, close after AUTH failure, one attempt for two callers.

Acceptance

• An unreachable host fails after the timeout with a timed out message.
• A failed AUTH closes the socket.
• Two concurrent connect() calls open one connection and one tunnel.