Appeal a rejected campaign

Appeals a rejected 10DLC campaign with the carrier registry. Only a registration that reached campaign creation can be appealed; a brand-level rejection should be fixed and re-verified instead. On success the registration returns to pending.

Content rejections (e.g. an opt-in flow without a verifiable form link, or unrealistic samples) should be FIXED in the same call: pass the corrected messageFlow / sample1 / sample2 and the campaign is updated before the appeal is filed, so the reviewer sees the new content. The current content is on GET /v1/sms/registrations/{id} (campaignContent).