fix(server): prevent stale terminal attach errors on session switch

[andrewting19]

Problem

Switching from a remote SSH session back to a local session could surface a global terminal error in the UI. Stale async SSH attach/start failures were reported after the selection changed, often with sessionId=null.

Fix

• Add a per-WebSocket attach sequence and ignore stale attach/detach completions.
• Report terminal attach/start errors against the requested sessionId (not ws.currentSessionId at failure time).
• Guard terminal output from replaced proxies.
• Harden Pty/Ssh proxy start/dispose races and make SSH attach use tmux new-session -A to avoid duplicate-session failures.

Testing

• bun run lint
• bun run typecheck
• bun run test