feat(kyc): consume context param from KYC/registration exceptions

[davidleomay]

Summary

• Parse optional context field from KycLevelRequiredException and RegistrationRequiredException API error responses
• Carry context through failure states → route navigation → KycPageManager → KycCubit → service layer
• Pass ?context=RealunitBuy (or RealunitSell) as query param on GET /v2/kyc and PUT /v2/kyc
• No local step-filtering logic — the API is the authority, the app passes through what it receives

Flow: API throws exception with context → cubit stores in failure state → button passes via route extra → KycPageManager forwards to KycCubit → service appends query param → API returns only relevant steps

Files changed (18)

Exceptions: buy_exceptions.dart — added nullable context field to both exception classes

States: buy_payment_info_state.dart, sell_payment_info_state.dart — added context to failure states + Equatable props

Cubits: buy_payment_info_cubit.dart, sell_payment_info_cubit.dart — pass e.context to failure states

Navigation: sell_button.dart, payment_additional_action_needed_button.dart — pass state.context via route extra

Route + page: router_config.dart, kyc_page_manager.dart — extract extra as kycContext, forward to cubit

KYC cubit: kyc_cubit.dart — store _kycContext, sticky across re-checks, forward to service calls

KYC service: dfx_kyc_service.dart — accept context param, append as query param on GET/PUT

Tests (7 files, +307 lines): exception parsing, state equality, cubit propagation, service query params, cubit sticky forwarding

Test plan

• flutter analyze — 0 new errors (pre-existing i18n issues only)
• api_exception_test.dart — 8/8 pass including new context parsing tests
• State props assertions updated for new context field
• Cubit tests verify context propagation from exceptions
• Service tests verify ?context= query param present/absent
• KycCubit tests verify context forwarded to service and sticky across re-checks
• Remaining test files blocked by pre-existing radioButtonUnselectedLabel i18n compilation error on develop