test: buy_payment_info_cubit + balance_service extensions (+14 tests)

[TaprootFreak]

Summary

Stage 16 of the coverage push.

File	Cases (new)
`buy/cubits/buy_payment_info/buy_payment_info_cubit.dart`	10 (new file)
`packages/service/balance_service.dart`	4 (appended to existing test)

What each file covers

• buy_payment_info_cubit: initial `BuyPaymentInfoInitial`; happy CHF path emits Success; amount below 100 CHF minimum → `MinAmountNotMetFailure` and the service is NOT called; EUR minimum is scaled by `getChfToEurRate` and ceil'd (e.g. 100 × 0.92 → 92); EUR amount above scaled minimum proceeds to the service; empty amount string is treated as 0; comma decimal separator normalised to dot (e.g. `'300,75'` → 301); `KycLevelRequiredException` → `Failure(kycRequired, requiredLevel)`; `RegistrationRequiredException` → `Failure(registrationRequired)`; generic exception → `Failure(unknown)`.
• balance_service (4 new cases on top of the existing 2): skips saving when the response JSON has no `balance` field; catches a non-numeric balance string and skips saving (production code does `BigInt.parse` inside try/catch); `getBalance` delegates straight through to `BalanceRepository.getBalance`; `cancelSync` is a safe no-op when called before `startSync`.

Notes

• `buy_payment_info_cubit` reads `DFXPriceService.getChfToEurRate` only for the EUR branch, so the CHF tests don't need to stub it.
• The cubit's source file is NOT touched by PR fix: keep dashboard buy actions visible #321 — only the underlying service is — so this PR is conflict-free.

Test plan

• `flutter analyze` on the two changed files — clean
• `flutter test` — 16 / 16 passing locally (12 in buy_payment_info_cubit + 6 in balance_service incl. the 2 pre-existing)
• CI green