fix: close sockets without synthetic error

[Haihan-Jiang]

Change Description

Fixes #576.

CloudSQLInstance.close() still destroys every tracked socket so shutdown can clean up active handles, but it no longer passes a synthetic CloudSQLConnectorError into socket.destroy(). Passing that error causes Node to emit an asynchronous socket error event; after drivers such as pg have drained a pool and removed socket listeners, that event can become an uncaught exception during intentional connector shutdown.

The added unit test verifies close-time socket teardown calls destroy() without an error argument.

Checklist

• Make sure to open an issue as a bug/issue before writing your code
• Ensure the tests and linter pass
• Appropriate documentation is updated (not necessary)

Validation

• npx tap -t0 -c test/cloud-sql-instance.ts
• npm run lint
• npm run compile
• npm test