- Introduce a `FeeRateFormat` enum and change `CFeeRate::ToString()`
to use it for `BTC/kvB` vs `sat/vB` output formatting.
- Handle all enum values, hence remove default case in `CFeeRate::ToString()`
and `assert(False)` when a `FeeRateFormat` value is not handled.
- Keep `FeeEstimateMode` focused on fee estimation behavior by removing fee rate format
values from `FeeEstimateMode`.
- Update all formatting call sites and tests to pass `FeeRateFormat` explicitly, separating fee rate format
from fee-estimation mode selection.
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