Reply to a message

Sends a threaded reply or a reaction targeted at an existing message. **The sender is derived from the target message.** The reply goes out from the Dial number the target message belongs to, to the other party of that message — there is no `to` or `fromNumberId`. A reply always stays in the conversation the target message is part of. The request carries **exactly one** of: - `body` — a text reply. On iMessage numbers it is delivered as a native threaded reply (quoting the target message) when the recipient supports threads, and as a regular message otherwise. On standard numbers it is delivered as a regular SMS. - `reaction` — a reaction to the target message: one of the six reaction names (`love`, `like`, `dislike`, `laugh`, `emphasize`, `question`) or a **single emoji** (one visible symbol — skin-tone and multi-person emoji count as one; anything else is rejected with `400`). On iMessage numbers the reaction is delivered natively (a Tapback). When the recipient can only receive SMS — including everything sent from standard numbers — an **emoji** reaction is delivered as a regular message whose body is the emoji, and a reaction **name** is rejected with `400` (names have no SMS rendering; send an emoji instead). **Targets.** Any message on the account can be targeted, with one current restriction: on iMessage numbers the target must be an **inbound** message — replying to your own sent messages isn't supported on those numbers yet. Standard numbers accept targets in both directions. A **reaction cannot itself be the target** — replying or reacting to a reaction is rejected with `400`. The created message is recorded like any outbound message and billed the same as Send a message. Its `replyToId` points at the target message; `reaction` carries the reaction string when one was sent (a reaction delivered natively has an empty `body`; one delivered as a regular message carries the emoji in `body` too). Not idempotent — retrying a failed request can send a duplicate reply. On an ambiguous failure, confirm via List messages before re-sending.