Use bash as the default shell for SSH connect sessions

[anton-107]

Summary

• Client-side fix: When no remote command is specified, spawnSSHClient now explicitly runs /bin/bash -l with -t for PTY allocation instead of relying on the default login shell from /etc/passwd (which is /bin/sh on Databricks compute images).
• Server-side fix: Before starting sshd, the server attempts to update /etc/passwd to set bash as the login shell for the current user. This covers IDE connections (VS Code, Cursor) that manage their own SSH sessions and don't go through spawnSSHClient.
• Both fixes degrade gracefully: the client-side fix only applies when no additional args are passed, and the server-side fix logs a warning and continues if /etc/passwd can't be modified.

Test plan

• Verified databricks ssh connect --cluster=<id> opens a bash shell (tested manually)
• Verify databricks ssh connect --cluster=<id> -- ls -la still works (additional args passthrough)
• Verify IDE connections (VS Code/Cursor) get bash after a fresh server start
• Verify sftp still works for IDE file operations

This pull request was AI-assisted by Isaac.

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @ilia-db -- recent work in experimental/ssh/internal/client/, experimental/ssh/internal/server/

Eligible reviewers: @andrewnester, @denik, @pietern, @renaudhartert-db, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}