Make modal submit message optional

[biast12]

Description

Update ModalSubmitInteraction so message is Option<Message> instead of required. This matches Discord payload behavior where message is only present for modals opened from components, and absent for slash command-opened modals.

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

• Run a slash command that opens a modal

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works