fix: propagate SSH authentication errors to the UI

[eduardomozart]

📋 Description

This PR fixes an issue where users receive no visual feedback when an SSH connection fails due to authentication issues. While the backend correctly logs the failure, the error was not being properly bubbled up or handled by the frontend, resulting in a silent failure state.

When the ssh2 library emits an authentication failure, the error is caught in controllers/serverSession.js, but the response is either not sent or is sent in a format the UI doesn't recognize as a displayable error.

[0] 2026-02-24 10:43:21 [error] [controllers/serverSession.js:92] Failed to create connection for session sessionId=e7d6f9af-5bad-4071-a8c6-9a14feb154ae error=Invalid username stack=Error: Invalid username
[0]     at Client.connect (/home/vaultwarden/git/Nexterm/node_modules/ssh2/lib/client.js:203:13)
[0]     at createSSHClient (/home/vaultwarden/git/Nexterm/server/lib/ConnectionService.js:124:9)
[0]     at createSSHConnectionForSession (/home/vaultwarden/git/Nexterm/server/lib/ConnectionService.js:58:23)
[0]     at process.processTicksAndRejections (node:internal/process/task_queues:105:5)

Fix

• Updated server/controllers/serverSession.js to ensure that when a connection failure occurs, a proper error response (e.g., 500 or 401) is returned with a descriptive message.
• Updated the frontend connection handler to catch this specific error and trigger a notification toast/alert.

How To Test

• Attempt to open an SSH connection using invalid credentials (wrong password or incorrect private key).
• Before fix: Errors only appear in server logs.
• After fix: The UI should immediately display an error message such as: "SSH Connection Failed: All configured authentication methods failed."

🚀 Changes made to ...

• 🔧 Server
• 🖥️ Client
• 📚 Documentation
• 🔄 Other: ___

✅ Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have looked for similar pull requests in the repository and found none
• This pull request does not contain translations (translations are managed through Crowdin)

🔗 Related Issues

Closes #1168

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[tcatlas]

May be a duplicate of #1053, which covers all connections not just SSH.