Interval scheduling UI missing in 9.x

## Bug Description
Report from Community Slack: https://elasticstack.slack.com/archives/C06U8G8NJBY/p1770128013639309

```
Running into a limitation with the jira content connector incremental sync scheduler. We need to sync every 3 hours. But cannot seem to figure out how we can set that schedule with the options available. We appear to be limited to choosing from:

5, 10, 15, or 30 minA certain hour each day (but only get to choose one)A certain time each day (but only get to choose one) (this seems to be the same as last bullet)One day and time each week (again only get one choice)One day and time a month (also only one choice)A day and time within a certain month (but again only one choice)

Are we missing something? Is there a way to schedule it to run every 3 hours?
```

it seems that the UI is missing the interval (cron-like) scheduling feature in 9.x.

Setting the [schedule via API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-connector-update-scheduling) was a sufficient workaround.

### To Reproduce
1. create a connector in 9.x
2. try to schedule a sync for every 3 hours
3. be stuck

## Expected behavior
The interval scheduling UI should be accessible.

Start by reproducing the scheduling flow described for a connector in 9.x and compare its available options with the connector update scheduling API linked in the issue. The work is done when the UI exposes interval scheduling, including a three-hour interval, and the behavior is covered by the relevant UI tests if they exist.

Written by the indexing model from the issue text.