Resolve all todos when user submits an MR review

What does this MR do and why?

We changed todo resolution in %17.2 to resolve todos one discussion at a time. This negatively impacted the flow for submitting code reviews though, as those primarily behave as discussion notes internally. This MR restores the previous behavior: when submitting a review, all of the current user's todos for the MR are resolved.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure the multiple_todos feature is enabled

   Feature.enable(:multiple_todos)

2. Create a merge request
3. Mention a user (let's call him Bob) in the comments
4. Request a review from Bob
5. Impersonate as Bob
6. View the to-do list – confirm there are 2+ todos for the MR
7. Submit a review for the MR as Bob
8. Confirm that all todos for the MR are resolved