test(kyc): financial DTOs + KycLevelDto (+10 tests)

[TaprootFreak]

Summary

Stage 39 of the coverage push. Pure-DTO tests for the KYC financial-data wire surface plus the `KycLevelDto` wrapper that ties `KycLevel` to a list of `KycStepDto`.

Cases

Target	Cases
`QuestionType.fromValue`	2 — four documented wire values (`Confirmation` / `SingleChoice` / `MultipleChoice` / `Text`); throws `ArgumentError` on unknown
`KycFinancialOption.fromJson`	1 — key + text
`KycFinancialCondition.fromJson`	1 — question + response
`KycFinancialQuestion.fromJson`	2 — happy path (description + options + conditions); description / options / conditions all optional
`KycFinancialResponse`	1 — `fromJson` + `toJson` round-trip
`KycFinancialOutData.fromJson`	2 — with responses; absent `responses` defaults to `[]`
`KycLevelDto.fromJson`	1 — `kycLevel` + nested `KycStepDto` list

What's pinned

• `QuestionType.fromValue` is opt-in strict (throws on unknown wire values) unlike the support enums which fall back to a sentinel — pinned by both the happy path and the negative case.
• `KycFinancialQuestion` treats `description`, `options` and `conditions` as fully optional — wire-side `null` survives as Dart `null`, not an empty list.
• `KycFinancialOutData.responses` defaults to `[]` when the key is absent (not `null`), distinguishing it from the question-side optionals.

Test plan

• `flutter test test/packages/service/dfx/models/kyc/kyc_financial_data_dto_test.dart` — 10 pass
• `flutter analyze` clean on the new file
• CI green