Suggest to user which SSH extension they need to install if missing

https://github.com/coder/vscode-coder/pull/536#discussion_r2153351637

This PR adds graceful handling of missing SSH extensions, and we could probably add a little code here to figure out which extension they need to install based on the editor they're using (VSCode handles it automatically, but downline forks like e.g. Windsurf may not)