fix: SFTP server subsystem sends exit-status and EOF before channel close

[Luwdo]

Problem

When using ssh2 as an SSH server, the server-side SFTP class's destroy() method sends channelClose without first sending exit-status or channelEOF. This causes SCP clients to report exit code -1 (or 0xffffffff), even when the SFTP transfer completes successfully.

Modern OpenSSH's scp uses the SFTP subsystem by default (since OpenSSH 9.0), so this affects any SCP operation through an ssh2-based server.

Root Cause

The SFTP class extends EventEmitter, not Channel. The Channel class handles exit-status properly via its onFinish handler (which calls eof() then close()), but SFTP.destroy() skips straight to channelClose:

// Before
destroy() {
    if (this.outgoing.state === 'open' || this.outgoing.state === 'eof') {
      this.outgoing.state = 'closing';
      this._protocol.channelClose(this.outgoing.id);
    }
}

In contrast, OpenSSH's sftp-server process exits with code 0 when done, which causes the server to send exit-status(0) → SSH_MSG_CHANNEL_EOF → SSH_MSG_CHANNEL_CLOSE on the channel.

Fix

When running in server mode, send exitStatus(0) and channelEOF before channelClose in destroy():

destroy() {
    if (this.outgoing.state === 'open' || this.outgoing.state === 'eof') {
      if (this.server && this.outgoing.state === 'open') {
        this._protocol.exitStatus(this.outgoing.id, 0);
        this._protocol.channelEOF(this.outgoing.id);
      }
      this.outgoing.state = 'closing';
      this._protocol.channelClose(this.outgoing.id);
    }
}

Testing

Verified with:

• scp (SFTP mode, default) — now returns exit code 0 instead of -1
• scp -O (legacy mode, uses exec) — unaffected, still works
• Mutagen file sync agent auto-install via SCP — now succeeds without manual agent installation
• Normal SFTP file operations (read, write, mkdir, stat, etc.) — unaffected