fix(aws): family expiry report accuracy and non-size-flexible RDS engines (follow-up to #341) #342

Two non-blocking follow-ups to #341 (ApplyFamilyNUSizingRDSWithExpiry). Both were raised by CodeRabbit on the PR; the merge-gate review triaged them as non-blocking.

1. FamilyExpiryReport.ExpiringNU overstates the deduction. The godoc says it is the NU "deducted from family coverage", but adjustFamilyCoverageForExpiry adds every expiring unit, even when the min() leaves CoveredNU unchanged (active RIs exceed covered usage). Example: 40 NU covered, 80 NU active, 16 NU expiring reports ExpiringNU=16 with 0 deducted. This affects only the diagnostic or log value; sizing is correct. Fix: either accumulate pre - post per family, or reword the field doc to "expiring NU considered".
2. Non-size-flexible RDS engines are pooled by family. For SQL Server and Oracle License Included, AWS gives no size flexibility, but the expiry adjustment (like the existing family sizing) treats remaining smaller RIs as able to cover larger expiring usage. With the capped formula max(Covered-exp, min(Covered, active-exp)) this errs toward a smaller deduction, so it can under-buy for these engines; it cannot over-buy. Fix: keep those engines at size-pool granularity in both the family sizing and the expiry deduction.

Acceptance: a test per item that fails before the fix.