Text-only Matrix messages sent via the send_message engine (hermes send,
cron deliver: matrix) arrived unencrypted (red padlock) in E2EE rooms.
Media sends already routed through the mautrix adapter and encrypted fine,
but text-only sends took the raw-HTTP standalone_sender_fn path, which
never encrypts.
Route ALL Matrix sends through _send_matrix_via_adapter so text is
encrypted too. The adapter reuses the live gateway's E2EE session when
available (#46310) and falls back to an encryption-aware ephemeral adapter
for standalone/cron contexts. The registry standalone_sender_fn stays
registered for the contract; it is simply no longer reached for Matrix.
Salvaged from PR #20259 onto current main (the original patched the
pre-#41112 _send_matrix branch, which had since moved to the plugin's
standalone path).
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>
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