Update extract validate for time point and time average variables in same stream

Extract validation is crude. It cannot tell the difference between time mean and timepoint data extracted from mass in the same stream. This was found when processing for cmip7 using:
```
atmos/pr_tavg-u-hxy-u@3hr:ap8 # embargoed
atmos/pr_tpt-u-hxy-u@3hr:ap8 # embargoed, priority=medium

```

Start by reproducing extract validation with the two listed CMIP7 stream variables, atmos/pr_tavg-u-hxy-u@3hr:ap8 and atmos/pr_tpt-u-hxy-u@3hr:ap8. Locate the extract validation entry point and determine how it currently handles time-mean and time-point data in one stream; done means the two variable types are distinguished correctly.

Written by the indexing model from the issue text.