* fix(relay): authorize relay-delivered events by delivery, not source.platform
The #52190 upstream-authz fix keyed _is_user_authorized off
source.platform via _adapter_authorization_is_upstream(source.platform).
But a relay *message* inbound carries the UNDERLYING platform
(source.platform == discord/telegram/...), NOT Platform.RELAY, because
ws_transport._event_from_wire maps the connector's wire payload
(platform="discord") straight onto SessionSource for session-keying and
egress. The relay adapter is registered only under Platform.RELAY, so
adapters.get(Platform.DISCORD) misses, the trusted-upstream branch is
skipped, and the user hits the env-allowlist default-deny:
WARNING gateway.run: Unauthorized user: <id> (<name>) on discord
(Live staging bug: alpha tester linked successfully, then every
follow-up DM was silently dropped.)
Fix: the authentic trust signal is that the event was delivered over the
per-instance-authenticated relay WS, not which platform it underlies. Add
a wire-INVISIBLE SessionSource.delivered_via_upstream_relay flag, stamped
by the relay transport in _event_from_wire, and authorize on it. The flag
is excluded from to_dict/from_dict so a peer can neither forge it across
the wire nor have it restored from persistence. The existing adapter-flag
check is retained for events whose source.platform IS Platform.RELAY
(interaction-passthrough). A direct Discord event on a multiplexing
gateway (direct + relay adapters) is unmarked and still default-denies.
* fix(relay): use identity check on delivery marker to avoid MagicMock fail-open
A MagicMock() source (used by test_signal.py and other gateway tests) auto-
vivifies source.delivered_via_upstream_relay as a truthy Mock, which a bare
truthiness check would treat as authorized — flipping
test_signal_in_allowlist_maps from False to True. The marker is a real bool on
SessionSource, so check 'is True' explicitly: refuses to authorize any non-bool
stand-in, defensive against accidental fail-open.
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