SMS and both WeCom adapters now split long replies, and their chunk loops
returned a plain failure when a later chunk was rejected. The base send
path cannot tell that from a total failure, so _send_with_retry retried the
whole payload or sent the 3500-char plain-text fallback and reported
success: the recipient got the head twice and never got the tail.
The three loops now share gateway.platforms.helpers.send_chunks(), which
marks a failure after delivered chunks with the existing partial_overflow
contract (the one Telegram and Discord set and _is_partial_delivery reads).
The retry and plain-text fallback already refuse to re-send a partial
delivery, so the caller gets an honest failure and nothing is duplicated.
Resuming the undelivered tail is #124295's scope.
c8cde58273