flux-cancel argument parsing issues

[grondo]

@pyrsq noticed some argument parsing strangeness with the flux cancel command. It is likely other Python-based commands are subject to the same quirks:

# Works as expected
hetchy1001 ~> flux cancel --foo
usage: flux cancel [OPTIONS] [JOBID...]
flux-cancel: error: unrecognized arguments: --foo

# Works as expected
hetchy1001 ~> flux cancel --foo=a
usage: flux cancel [OPTIONS] [JOBID...]
flux-cancel: error: unrecognized arguments: --foo=a

# Does not see it as an invalid optional argument
hetchy1001 ~> flux cancel --foo="a b"
usage: flux cancel [OPTIONS] [JOBID...]
flux-cancel: error: argument JOBID...: invalid JobID value: '--foo=a b'

# Extra weird (something about that period at the end leads to it being a valid job ID as far as parsing is concerned)
hetchy1001 ~> flux cancel --foo="a ."
flux-cancel: ERROR: f1: unknown job id

[grondo]

The final issue above has been fixed by #7600.

The remaining issues are indeed due to an argparse quirk, which I think was described in this issue from 2014 and has yet to be resolved: python/cpython#66623

As noted in the issue, the problem can be shown in this simple reproducer:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('jobids', nargs='*')
# either of these should cause program to exit with `unrecognized arguments`:
print(parser.parse_args(['--foo="foo bar"']))
print(parser.parse_args(['--foo="foo+bar"']))

$ /usr/bin/python3 x.py
Namespace(jobids=['--foo="foo bar"'])
usage: x.py [-h] [jobids [jobids ...]]
x.py: error: unrecognized arguments: --foo="foo+bar"
$ python3 -V
Python 3.13.2
$ python3 x.py
Namespace(jobids=['--foo="foo bar"'])
usage: x.py [-h] [jobids ...]
x.py: error: unrecognized arguments: --foo="foo+bar"

I'm not sure how to work around this yet.