refactor(cli): simplify device ops with unified _require_device helper

- Extract _require_device() and _write_local() helpers to eliminate
  duplicated connection-check boilerplate across all device methods
- Simplify unpatch/info/test_serial/file_list/file_stat/file_download/
  mem_read/mem_write/mem_dump/serial_send/serial_read methods
- Fix E741 lint warnings (ambiguous variable name 'l' -> 'ln')
- Remove unused URLError import from server_proxy.py
- Add launch_server/ensure_server tests to test_server_proxy.py
- Add proxy serial/connect/disconnect tests to test_cli_coexistence.py
- Add PortLock context manager/is_locked tests to test_phase_completion.py
- Coverage: 84.3% -> 85.1% (server_proxy: 66% -> 95%)

Author: FASTSHIFT