Post-publish stage — reply to a buyer/inquiry message (POST /messages/reply).
Sends a real message and queues a real email notification to the recipient,
so the text you send is final — treat this as a live send, not a draft.
ALWAYS improve the user's drafted reply before sending: fix grammar,
spelling, punctuation, capitalization, and formatting, and tighten wording
for a clear, professional tone — while preserving their meaning and intent.
Send the corrected version, never the raw draft. When the user says "reply
saying X", treat X as a rough draft to polish, not literal text to send
verbatim.
Requires ``to_user_id`` (the counterparty), ``message``, and ``access_token``.
For a threaded reply pass the original ``thread`` verbatim and omit
``subject`` (the server prefixes "Re: <original subject>"). To find
``to_user_id``, open the conversation with `beycome_messages` and take the
participant id that is not the signed-in owner.
Success is 201 with ``data.id`` set to the new message id. Failures come back
as 400 with a plain message: "User not found." (bad ``to_user_id``), "You
cannot send a message to yourself." (``to_user_id`` is the sender), or
"Invalid parameters" (e.g. empty/over-long ``message``). A 401 means the
token is no longer valid — re-authenticate with `beycome_signin_start` then
`beycome_signin_verify`.