FIX: clear uploads after successfully posting new PM

[SamSaffron]

This PR addresses a bug where uploads weren't being cleared after successfully posting a new private message in the AI bot conversations interface. Here's what the changes do:

Main Fix:

• Makes the prepareAndSubmitToBot() method async and adds proper error handling
• Adds this.uploads.clear() after successful submission to clear all uploads
• Adds a test to verify that the "New Question" button properly resets the UI with no uploads

Additional Improvements:

1. Dynamic Character Length Validation:

   • Uses siteSettings.min_personal_message_post_length instead of hardcoded 10 characters
   • Updates the error message to show the dynamic character count
   • Adds proper pluralization in the localization file for the error message

2. Bug Fixes:

   • Adds null checks with optional chaining (link?.topic?.id) in the sidebar code to prevent potential errors

3. Code Organization:

   • Moves error handling from the service to the controller for better separation of concerns