Drawing lots for absolute majority

[oliver3]

What & why

• Resolves Backend: process drawing lots for absolute majority reassignment (P9) #3273
• Implements using drawing lots for absolute majority (P 9) in the apportionment crate

Implementation details:

• Update reassign_residual_seat_for_absolute_majority() to try and use a list from lists_drawn when drawing lots is needed
• Split ListDrawingLotsVariant::AbsoluteMajority into two variants, corresponding to a seat that is reassigned from a previous largest remainder or highest average seat change
• Add assign_to field to the AbsoluteMajorityDrawingLots variant details to be used in the frontend
• Add ApportionmentError::InvalidState variant and use it when a previous seat change is not one of the expected variants
• Helper functions and traits to keep the line count of reassign_residual_seat_for_absolute_majority() within acceptable limits

How to test

Unit tests are adjusted accordingly, frontend will be built in later issues from #788

Reviewer notes

Start in backend/apportionment/src/seat_assignment/mod.rs to see the core change in reassign_residual_seat_for_absolute_majority() and what the results are in the tests below. Then check what other changes were needed to make this happen.

[codecov]

Codecov Report

❌ Patch coverage is 79.66102% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (42350ea) to head (be1f322).
⚠️ Report is 1 commits behind head on main.

[oliver3]

Patch coverage is pretty low, I will add some tests to cover helpers and conversion of apportionment from/to abacus crate structs.