createMediaProtocolHandler() set only the session token / bearer on the
/api/files/stream request. The descriptor it resolves now carries the
connection's extra gateway headers, but the handler never read them, so
attachments in session history from a header-gated remote still bounced off
the access proxy even though every fetchJsonForBackend() call got through.
Merge connection.headers into the media request before auth, letting the
forwarded range/cache negotiation headers win, and pin it on both the
token and the OAuth cookie-session legs.
Part of #112072
c902f50efb