[MEET-560, 561, 562] Fix meetings API bugs

[mrmir]

Ticket

https://community.openproject.org/wp/MEET-560
https://community.openproject.org/wp/MEET-561
https://community.openproject.org/wp/MEET-562

What are you trying to accomplish?

Fix API bugs

What approach did you choose and why?

• For 560, I updated the state and upcoming/past determination via the API to always match the web versions. I'm not sure why they were not based off of the same thing in the first place
• For 561, I updated the contract to allow switching state when closed as well. This issue was never run into via the web version because we update state directly without going through the contract. Not sure if that needs to be handled separately, but it is out of scope for now imo
• For 562, I changed the agenda_items response to also include backlog agenda items. These were previously excluded since for the web version we handle those lists of items separately, but such a distinction doesn't really make much sense for the API response

Merge checklist

• Added/updated tests
• Added/updated documentation in Lookbook (patterns, previews, etc)
• Tested major browsers (Chrome, Firefox, Edge, ...)

[github-actions]

Warning

Flaky specs

• rspec ./spec/features/projects/lists/filters_spec.rb[1:6:1]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #23889, linked for reference only):

- `rspec ./spec/features/projects/lists/filters_spec.rb[1:6:1]`

Treat this as a standalone task, unrelated to PR #23889. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #23889 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @mrmir to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @mrmir, and request a review from @mrmir.
On every commit, set @mrmir as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.