GitLab

- Refactor OpenAI Messages() routing: pre-compute dispatch model using
  resolveOpenAIMessagesDispatchMappedModel + NormalizeOpenAICompatRequestedModel
  instead of try-fail-retry pattern with gin context passing
- Remove openai_messages_fallback_model context anti-pattern
- Use effectiveMappedModel directly for forward default mapped model
- Add 3 subscription group tests covering all branch paths:
  _Blocked (no active subscription → SUBSCRIPTION_REQUIRED),
  _RequiresRepo (nil repo → SUBSCRIPTION_REPOSITORY_UNAVAILABLE),
  _AllowsActiveSubscription (valid subscription → success)