reject missing choice value

[aouxwoux]

what changed

make an option with choices(...).nargs(1) reject the option when it appears without a value, and add a regression test.

why

when choices are configured, consume uses the number of matching values as the effective maximum. zero matching values was then treated as a zero-argument option, which bypassed the configured minimum and allowed --color to succeed with an empty value.

this keeps the zero-argument path for options whose configured minimum is zero, while allowing the normal minimum-argument validation to reject missing values.

checks

• git diff --check passes.
• local cmake configuration could not run because nmake and a c++ compiler are not installed in this checkout.

fixes #431