Teams overrode send_image/send_image_file but not send_video, send_voice,
or send_document — so when the gateway dispatched a video/voice/document
reply to a Teams chat it fell through to the base-class text fallback and
sent the local file path as plain text (same broken-UX class as the LINE
URL-image gap in #49298).
Extract the existing send_image attachment logic into a shared
_send_media_attachment helper (remote URL by reference, local file as a
base64 data URI, MIME guessed from the path) and route all four media
kinds through it. 5 new tests cover remote-URL, local-file base64,
no-app, and missing-file paths.
5f55f0ff85