Body shape depends on the tackle-operation-id header. The body is parsed
differently for each operation: acceptInvitation reads only crmId, and
rejectInvitation reads only rejectionReason. Each property is optional at
the schema level and only meaningful for the matching operation; properties
for the other operation are ignored.

Body requirement by operation:

• For acceptInvitation, the request body is fully optional. An empty body,
  {}, or {"crmId": "..."} are all accepted; an empty body skips body
  parsing entirely.
• For rejectInvitation, the request body must be a present, non-empty JSON
  object — at minimum {}. An empty (zero-length) body fails to parse and
  surfaces as a 500 Internal Server Error, not a 400 Bad Request. Always
  send at least {} (or include rejectionReason to propagate a reason to
  AWS Partner Central).