Implement Vipps API Client with token exchange and userinfo endpoints

Use Dart's http package (or dio) with a custom HttpClient abstracted behind an interface for testability. Model all requests as value objects: VippsTokenRequest, VippsUserinfoRequest. Parse responses into typed Dart classes with fromJson factories — never access raw map keys outside the model. Compute expiresAt = DateTime.now().add(Duration(seconds: expiresIn)) at parse time.

The refresh token flow should check if the refresh token itself is expired before making the network call. PKCE code_verifier must be received from the caller (generated by the deep link/OAuth layer) — the client is stateless. Register as Provider taking VippsEndpointConfig as a dependency. Consider wrapping the HTTP client in a thin interceptor that strips sensitive headers from debug logs.

Unit tests using flutter_test with a mock HTTP client (MockClient or dio interceptor). Test cases: (1) successful token exchange returns correct VippsTokenResponse fields; (2) successful userinfo returns typed response with sub and nin; (3) HTTP 400 maps to InvalidGrantFailure; (4) HTTP 401 maps to UnauthorizedFailure; (5) network timeout maps to NetworkFailure; (6) HTTP 500 maps to ServerFailure; (7) missing nin in userinfo response maps to MissingPersonnummerFailure. Assert no raw exceptions escape. Assert User-Agent header is present in all requests.