Follow-up to the reply-reference fix: `_make_discord_adapter` used to return
the raw fetched `Message` as the expected reference, but the adapter now
wraps it via `ref_msg.to_reference(fail_if_not_exists=False)` so Discord
treats a deleted target as 'send without reply chip'. Update the fixture
to return the MessageReference sentinel so the 4 chunk-reference-identity
tests assert against the right object.
No production behavior change; only aligns the stale test fixture.
eabe14af1c