The relay text lane stamps `SessionSource.profile` from the wire frame
(#60586), but `PassthroughForward` had no profile field, so a relayed
Discord slash-command/button/modal always landed in the default profile's
agent:main namespace even when the connector resolved a specific profile.
Add an optional `profile` to `PassthroughForward` (read off the wire in
`_passthrough_from_wire`) and stamp it on the interaction's SessionSource.
Absent on the wire → None → legacy routing, byte-identical for
single-profile gateways. Contract doc updated.
Salvaged from #61012 (pierrenode); two context conflicts resolved
(delivered_via_upstream_relay / _platform_by_chat landed on main).
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