Adds the contact identified by phoneNumber to the group identified by groupId. The request body chooses the enrollment mode:

• optInMode: "DIRECT" (default) — caller asserts prior consent. The contact is added as an active member immediately.
• optInMode: "CONFIRM" — the system sends an SMS confirmation to the contact. They land in pendingGroups[] until they reply YES.

Consent gate (cannot be overridden): HARD-opted-out contacts are rejected with 409 ContactHardOptedOut (33035). Contacts with PENDING memberships in groups other than the target are rejected with 409 ContactPending (33036).

Idempotency: if the contact is already active or pending in this group, the call is a no-op and returns the current contact state.