When auto-threading kicked in, the broadened backfill gate ran on the
freshly-created thread — but the thread has no prior context to fetch,
and the parent-channel reference passed to _fetch_channel_context would
have leaked unrelated context (see #31467).
Skip backfill when auto_threaded_channel is set. Also teach the
_FakeTextChannel / _FakeThreadChannel test doubles to expose a no-op
history() async generator so the broadened gate doesn't trip
AttributeError → discord.Forbidden (MagicMock) → TypeError in the
existing auto-thread tests. Add a regression test that asserts
auto-threaded messages do not trigger backfill.
b243afb68b