Bug 2066252 - Require a target milestone when adding checkin-needed-tb

[tobypilling]

• Prevent adding checkin-needed-tb without a target milestone for Calendar, MailNews Core, and Thunderbird bugs.
• Allow the keyword and milestone to be set in the same update.
• Do not block unrelated updates to bugs that already have the keyword.
• Add tests covering the validation behavior.

Testing

docker compose -f docker-compose.test.yml run --rm --build bmo.test test_bmo -q -f t/bmo/checkin-needed-tb-milestone.t

All 8 tests passed.

[dklawren]

I would rather not utilize a new hook and just use the already present evaluate_create() and evaluate_change() methods. I have submitted pr #2734 with the changes which I had already worked on. So you can see what I mean there. Much simpler.

[tobypilling]

Fantastic, thanks a lot!! Your implementation is simpler and handles the final bug state nicely, including setting the keyword and milestone in the same save. Closing this in favour of #2734.