fix(converter): prevent ColourConverter from returning non-Color types like tuples

[devbutlazy]

Summary

Fix #1488 => in ColourConverter.convert where certain classmethods (like holographic_style) return a tuple of colors instead of a single disnake.Color instance. Previously, this caused the converter to return a tuple instead of raising an error.

This PR adds a type check after invoking the color method and raises BadColourArgument if the returned value is not a single disnake.Color.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running uv run nox -s lint
  • I have type-checked the code by running uv run nox -s pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Enegg]

The fix you are proposing is a band-aid. The library shouldn't be invoking any non-factory methods of Color in the first place. Invoking them and only then checking the return type can lead to bugs in the future (what if a method was added that mutates some state?)

To properly fix the issue, we should have some kind of mapping of valid names to the methods, or at least a set to validate input beforehand.

[devbutlazy]

To properly fix the issue, we should have some kind of mapping of valid names to the methods, or at least a set to validate input beforehand.

If you provide such a set, I'd implement the proper issue fix.