Matrix media delivery was silently dropped by send_message because Matrix
wasn't wired into the native adapter-backed media path. Only Telegram,
Discord, and Weixin had native media support.
Adds _send_matrix_via_adapter() which creates a MatrixAdapter instance,
connects, sends text + media via the adapter's native upload methods
(send_document, send_image_file, send_video, send_voice), then disconnects.
Also fixes a stale URL-encoding assertion in test_send_message_missing_platforms
that broke after PR #10151 added quote() to room IDs.
Cherry-picked from PR #10486 by helix4u.
276ed5c399