test(dto): RealUnitSellPaymentInfoDto.fromJson (+3 tests)

[TaprootFreak]

Summary

Stage 49 of the coverage push. Final big sell-flow DTO that wasn't reachable from the payment_dtos aggregate file — it recursively pulls in `Eip7702Data` + `DfxFeesData` + `BeneficiaryDto` + `PriceStep`.

Cases

Target	Cases
`RealUnitSellPaymentInfoDto.fromJson`	3 — full happy-path wire shape; nested `Eip7702Data` DTO walked all the way down (one leaf per level pinned); integer wire values for `amount` / `exchangeRate` / `rate` widen to double

What's pinned

• The recursive `fromJson` chain actually parses every nested DTO. One leaf field per level (`relayerAddress` → `domain.chainId` → `message.delegate` → `amountWei`) is pinned so a regression to "stops at top level" surfaces here.
• `Currency.fromCode` is the wire→Dart conversion; `CHF` round-trips correctly.
• Integer-typed JSON values widen to double for the floating-point fields — pins the contract used by partner APIs that sometimes serialize whole-number rates as `int`.

Test plan

• `flutter test test/packages/service/dfx/models/payment/sell_payment_info_dto_test.dart` — 3 pass
• `flutter analyze` clean on the new file
• CI green