fix: better fix integer overrun when calculating parallelism for very long time range queries

[slim-bean]

What this PR does / why we need it:

#17428 didn't actually fix the issue, I had modified the test parallelism to 2048 when debugging but reverted it to the previous value in the test when cleaning up my PR.

Thanks to @rfratto for being curious here and digging a bit deeper.

The overflow was actually happening in the multiplication of the nanosecond duration of the query length and the query parallelism value.

This PR hopefully really fixes the issue by reducing the precision we use in this calculation, we don't need nanosecond precision for determining an approx weighed parallelism 👍

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

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• Documentation added
• Tests updated
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