`additional_user` value is both str & list

[CodeSpent]

...       
additional_mentions: discord.Option(str, "Additional user mentions (e.g., @user1 @user2)", default=[], name="mentions"),
...

You can see the option is a str but the default value is []. This is necessary currently because at user input we take in a comma-delimitted string of mentions (best effort, not intuitive), but functionally we use a list of user objects.

This isn't necessarily an issue to address in a vacuum, but we should reconsider the implementation here to make this more consistent.

In general the integration for multiple users being mentioned has not been well done yet. Welcome any recommendations on a better approach.