Create Child Client

Updates the editable fields of a visible client: `name`, `external_reference`, `metadata`, and `methodology_configuration`. Only the fields you send are changed; the others keep their current values. Sending `methodology_configuration` replaces the client's methodology options in full — send every option the client should keep, not just the ones that change. When the resulting list differs from the current one, the activities already stored for this client are recalculated with the new methodologies in the background; the response returns as soon as the configuration is written. The whole update is atomic: options that cannot be read as a list are rejected with `invalid_methodology_options` (`400`), and readable but disallowed options — an unknown `activity_type` or `methodology`, two defaults for one activity type, more than three per type — with `invalid_methodology_options` (`422`). Either way nothing is written, including the other fields sent in the same request. Re-parenting is not supported. A `parent_id` that would move the client — including `null`, which reads as a request to make it a root — is rejected with `parent_id_immutable` (`400`) and nothing is written. Sending the client's current `parent_id` is accepted and ignored, so you can read a client, change a field, and send the whole object back unchanged in shape. Requires this feature to be enabled for your account (contact us); otherwise this endpoint responds with `404`.