_auto_create_thread's dedup pre-seed (is_duplicate(str(thread.id))) stops
the echo MESSAGE_CREATE Discord fires for the starter (id == thread.id)
from re-running the request. This PR turned the tracker persist into
`await self._threads.mark_async(thread_id)` and placed it BEFORE the
pre-seed; to_thread always suspends, so the echo's handler could reach
_discord_message_admission -> is_duplicate during the os.replace window,
claim the id first, and rerun the starter. On main both statements were
sync, so no window existed.
Move the pre-seed (with its comment) directly after
`thread_id = str(thread.id)`; the await now follows it.
Other mark_async call sites checked (discord :4717 slash create-thread,
discord :6140 pre-dispatch, matrix :1438 create_thread, matrix :2083
inbound): nothing after those awaits relies on state a concurrent event
could claim first, so no further reorder.
PROOF: tests/gateway/test_discord_double_dispatch.py::
test_thread_starter_duplicate_dropped now installs a recording mark_async
that asserts thread.id is already in _dedup._seen when it is awaited.
Red with the pre-swap order (1 failed), green after; ruff clean,
check-windows-footguns clean, real import of the adapter from the
worktree ok, scripts/run_tests.sh on the PR's test files green.
b9ec39939b