add dedicated tests for --color parsing edge cases

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Add focused tests for --color parsing behavior in nx-cli.

Why

parse_requested_color supports multiple forms (--color=... and --color ...) and custom validation paths.
Dedicated tests improve confidence in CLI UX and error quality.

Scope

• in crates/nx-cli/src/main.rs tests:
  • cover --color=auto|always|never
  • cover --color <value>
  • cover missing value after --color
  • cover invalid value handling
• no CLI behavior changes unless needed to match intended current contract

Done when

• tests pass under cargo test -p nx-cli
• valid values parse correctly
• invalid/missing values return expected clap errors

Notes

Please keep this issue limited to color argument parsing tests.

Thanks again for considering contributing to Numax.