test(kyc): KycStep + KycSession DTO hierarchy (+12 tests)

[TaprootFreak]

Summary

Stage 31 of the coverage push. Wire-format DTOs for the KYC session hierarchy.

DTO	Cases
`KycStepDto.fromJson`	4
`KycSessionInfoDto` + `UrlType`	3
`KycStepSessionDto.fromJson`	1
`KycLevelDto.fromJson`	2
`KycSessionDto.fromJson`	2

What's pinned

• KycStepDto: full-step parse (name + type + status + reason + sequenceNumber + isCurrent); type / reason are optional (null on the wire stays null); `isCurrent` defaults to `false` when missing entirely (`??` fallback — pinned because callers rely on this); unknown step name throws (no silent fallback).
• KycSessionInfoDto / UrlType: url + UrlType parse; `UrlTypeExtension.fromJson` covers all four enum values (Browser/API/Token/None); unknown wire string throws `ArgumentError`.
• KycStepSessionDto: inherits all step fields; carries a nested `KycSessionInfoDto` parsed from `session`.
• KycLevelDto: level + step list; empty step list is allowed.
• KycSessionDto: extends `KycLevelDto` with an optional `currentStep` (null stays null).

Test plan

• `flutter analyze` clean
• `flutter test` — 12 / 12 passing locally
• CI green