Improve GROUP BY TIME when endtime - starttime < interval

When use GROUP BY TIME if endtime - starttime < interval currently the row result is rounding down, showing 1 row less.

now
(endtime - starttime) / interval = 5.3 -> 5 rows, currently we will show 5 rows of result. it rounds down

want
(endtime - starttime) / interval = 5.3 -> 6 rows, show 6 rows of result. it rounds up.

e.g., select avg(*) from root group by ( [now() -1h, now()), 7200s) will return 1 row

Search the repository for the GROUP BY TIME implementation and its query or aggregation tests, then reproduce the interval case from the issue: a one-hour range grouped by 7200s. Done means the 5.3-interval case produces 6 rows rather than rounding down to 5, with a regression test covering the behavior.

Written by the indexing model from the issue text.