kill flaky drains, wait for PS1 + long-timeout bailout, harden shell

[deathbyknowledge]

• Replace time-based drains with marker-based reads:
  • Only treat PS1 as completion (never PS2)
  • Regex-parse PS1 to extract exit code; centralize cleanup
  • Micro-poll while waiting; strip ANSI
• Add long-timeout bailout for incomplete input:
  • On overall timeout: send '\n' -> wait -> send ^D -> wait
  • Make ^D safe via set -o ignoreeof
• Startup path: run container with sleep infinity; attach via exec(TTY)
• Remove drain()/clean_terminal_output(); add strip_markers_and_extract_exit_code()
• Tests: stronger 200 assert msg; add base64 pipe case; formatting
• Misc: doc comments; slight stop behavior tweak (Stopped → NotStarted)
• Deps: add const_format and lazy_static
• Timeouts now return 504
• Already-stopped sandbox now returns NotStarted