Lift the on/off value parser to `manager_util.is_off_value` so the
prestartup hook and the new tests share one definition, and cover the
flag's contract:
- default is enabled (today's behavior is preserved bit-for-bit)
- PIPFixer.fix_broken short-circuits without spawning subprocess when
the flag is off
- PIPFixer.fix_broken still reaches subprocess when the flag is on
- make_pip_cmd is not gated — read paths (`pip list`, `freeze`, `show`)
the UI and security check rely on continue to work
- value parser accepts off/false/0/no/disabled (case- and
whitespace-insensitive) and treats unknown values as "on" so a typo
doesn't silently disable installs
