Classify register/complete "Invalid signature" by API code, not message text

[marassteiner]

Summary

RealUnitRegistrationService.completeRegistration decides whether a 400 is a content rejection (RegistrationRejectedException) or a generic ApiException by comparing error.message to the free-text string 'Invalid signature'. That mapping should use a stable API error code once the backend exposes one.

Evidence

• lib/packages/service/dfx/real_unit_registration_service.dart:215 — error.message != 'Invalid signature'
• test/packages/service/dfx/real_unit_registration_service_test.dart:315 — the test pins the same literal
• lib/packages/service/dfx/exceptions/api_exception.dart:15-21 already classifies structured codes (KYC_LEVEL_REQUIRED, REGISTRATION_REQUIRED)

Found while reviewing #910. That PR is correct to stop mapping this 400 to "check your entries"; the string match is only a brittle contract, not a regression of the #905 fix.

Suggested follow-up

1. Backend: return a stable code such as INVALID_SIGNATURE on POST /v1/realunit/register/complete.
2. App: classify on error.code (or a dedicated ApiException subclass via fromJson).
3. If a new exception type is added, enumerate it in exception_surface_test.dart in the same PR.

Not a merge blocker for #910: if the message text ever changes, the UI falls back to the previous generic failure path rather than blocking registration.