Ensure note is promoted to discussion within reply create transaction

Previously a note on a merge request would be committed to the database before its type is promoted to DiscussionNote. However, as we saw in #301237 (closed), if a user replies to an individual note on the merge request but the UPDATE to the note fails (e.g. due to a SQL timeout), the note will persist in the database but have the wrong discussion type.

This can block users from being able to merge a merge request because the backend will detect an unresolved discussion. However, the UI won't show any unresolved discussions because the first note will erroneously appear to be an individual note, which cannot be resolved since it's not a discussion.

To avoid this issue, we now promote the type within the save transaction so that the note will always persist with the right value.

SQL statements

Before

After

Closes #301237 (closed)