fix(discord): skip backfill for auto-created threads and update test fakes

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.

plugins/platforms/discord/adapter.py

@@ -4820,8 +4820,10 @@ class DiscordAdapter(BasePlatformAdapter):
             #     recovery)
             # DMs skip entirely because every DM message triggers the bot,
             # so the session transcript already has everything.
+            # Auto-threaded messages also skip — we just created the thread,
+            # there's nothing prior to backfill.
             _has_mention_gap = require_mention and not is_free_channel and not in_bot_thread
-            if _has_mention_gap or is_thread:
+            if (_has_mention_gap or is_thread) and auto_threaded_channel is None:
                 _backfill_text = await self._fetch_channel_context(
                     message.channel, before=message,
                 )