Fix: Update operate mode and associated tests

[brynpickering]

Fixes confusion from the tests as to whether operate mode works in v0.7 or not (it does).

I've also made some fixes to the code to deal with edge cases that appeared while I was cleaning up the tests.

Reviewer checklist

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.35%. Comparing base (b087580) to head (7bd052a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #628 +/- ## ======================================= Coverage 95.35% 95.35% ======================================= Files 24 24 Lines 3656 3656 Branches 801 733 -68 ======================================= Hits 3486 3486 Misses 82 82 Partials 88 88 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/628?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/628?src=pr&el=tree&filepath=src%2Fcalliope%2Fmodel.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL21vZGVsLnB5) | `94.86% <100.00%> (ø)` | |