Fix validation on notes added to diff files

What does this MR do and why?

• Resolves sub-task 5 of https://gitlab.com/gitlab-org/gitlab/-/issues/411329
• A new-type of comment was added in !121429 (merged), which allowed users to comment on a file in the MR instead of a line
• This MR adds error-handling to that comment-type. Follows up from !122849 (merged)

Screenshots or screen recordings

User Action	Before	After
Adding a draft comment	add-comment-now-before	add-comment-now-after
Adding a comment	add-to-review-after	add-to-review-after

How to set up and validate locally

1. Create a Merge Request with at least 1 file change and 1 image file added.
2. Add a validator to the Note and DraftNote since we don't have a lot of validations for comments.

# Add it to class DraftNote < ApplicationRecord and 
# class Note < ApplicationRecord in models/

validate :test_ui_validation, on: [:create, :update]

def test_ui_validation
    if note.include? 'TEST_VALIDATION'
      self.errors.add(:base,  _('it failed validation'))
    end
end

1. Enable feature flag comment_on_files
2. Navigate to the Changes section and add a comment to a diff file. The tooltip should say Comment on this file. Add TEST_VALIDATION as the comment, which should trigger the error message.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #411329