feat: extended days of supply unit tests with edge cases

[ReneSchroederLJ]

Description

• extended days of supply unit tests:
  • test for insufficient number of days
  • initial stock of 0
  • mixed own and reported deliveries

Pre-review checks

Please ensure to do as many of the following checks as possible, before asking for committer review:

• DEPENDENCIES are up-to-date. Dash license tool. Committers can open IP issues for restricted libs.
• Copyright and license header are present on all affected files
• If helm chart has been changed, the chart version has been bumped to either next major, minor or patch level (compared to released chart).

[tom-rm-meyer-ISST]

LGTM, this took me some time. Revisiting this, I'm not sure if we should reiterate the deliveries standard, as there will likely be spec gaps.

[tom-rm-meyer-ISST]

Please add CHANGLEOG.md entry

[tom-rm-meyer-ISST]

LGTM