Fix amortised cost calculations

[lcouzens]

Jira Ticket

COST-####

Description

The only line items types that need to use savingsplan_savingsplaneffectivecost instead of unblended when calculating amortised_cost today are SavingsPlanCoveredUsage and SavingsPlanNegation

Instead of adding all variants of line item types it makes more sense to only consider these two line item types instead.

Testing

Tested this against internal data comparing directly with the bill

Happy to share this directly with people if your interested.

Release Notes

• proposed release note

* [COST-####](https://issues.redhat.com/browse/COST-####) Fix some things

[lcouzens]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.2%. Comparing base (aabdbae) to head (4963922).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   94.2%   94.2%           
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  Files        370     370           
  Lines      31539   31539           
  Branches    3378    3378           
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  Hits       29700   29700           
  Misses      1196    1196           
  Partials     643     643