Fix validation in UI when adding comments in MR diff

What does this MR do and why?

• Resolves sub-task 3 of https://gitlab.com/gitlab-org/gitlab/-/issues/411329
• Displays error in the UI when there is an error adding a note in an MR diff - be it a draft note or a normal note
• Currently, we don't have that many validations on notes but this is a blocker to start displaying if the comment being posted by a user is considered spam.
• This MR only addresses adding comments, updating comments will be addressed in a separate MR

Screenshots or screen recordings

User Action	Before	After
Adding draft comment	adding-draft-comment-before	adding-draft-comment-after
Replying with a draft comment	replying-draft-comment-before	replying-draft-comment-after
Adding a comment	adding-comment-before	adding-comment-after
Adding a comment (image diff)	adding-comment-image-diff-before	adding-comment-image-diff-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

3. Try to add comment TEST_VALIDATION in the Changes section.

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