send_document() accepted reply_to but never passed it down, so attachments
always threaded from the cached per-address context (or not at all) even
when the caller named the message to reply to. The plain-text path
(_send_email) already honored it; the attachment path now does too.
The metadata half of #10131 (send_image rejecting metadata=) was already
fixed on main by the adapter parity pass. Diagnosis from #10131 and the
explicit-reply_to-wins shape from PR #10321 (which targeted the
pre-plugin path).
Fixes#10131
Co-authored-by: LeonSGP43 <cine.dreamer.one@gmail.com>
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