Fix wrong raw output during vastai attach ssh --raw cmd call

[borisermakof]

Hey folks!

This PR fixes two issues:

• Incorrect attach ssh command description.
• Missing proper JSON output when the --raw parameter is used.

While working with the vastai CLI, I noticed that my code could not parse JSON when running:

vastai attach ssh <params> --raw

This is because the command returned plain text, which is not useful for programmatic parsing.

Example before the fix:

❯ python vast.py attach ssh <id> <path>/keys/vastai_key.pub --raw  

"{'success': True, 'msg': 'SSH key added to instance.', 'key': '{"id": <id>, "user_id": null, "default": null, "public_key": "<keydata>", "private_key": null, "created_at": <date>, "deleted_at": null}'}  
null

The output is a string containing a plain text, not valid JSON.
There’s also an extra null line at the end. This forces me to do additional parsing/replacing before I can run json.loads(output).

Example after the fix:

❯ python vast.py attach ssh <id> <path>keys/vastai_key.pub --raw  

{
  "key": "{\"id\": <id>, \"user_id\": null, \"default\": null, \"public_key\": \"<key_data>\", \"private_key\": null, \"created_at\": 1754655150.416915, \"deleted_at\": null}",
  "msg": "SSH key added to instance.",
  "success": true
}

Now the output is valid JSON and can be directly passed to json.loads() without additional processing.