fix(gateway): prevent double dispatch of Discord messages via thread-starter dedup

When _auto_create_thread() creates a thread from a user message via
message.create_thread(), Discord fires a second MESSAGE_CREATE event
for the 'thread starter message'.  That starter message carries
message.id == thread.id and may arrive with type=default instead of
type=21 (thread_starter_message), so the existing type filter in
on_message does not catch it — triggering a second call into
_handle_message and thus a second agent run and response.

Fix: after _auto_create_thread succeeds and returns a thread, pre-seed
the dedup cache with str(thread.id) via self._dedup.is_duplicate().
The dedup cache is the same TTL-based MessageDeduplicator that already
guards against Discord RESUME event replays.  Calling is_duplicate()
marks the ID as seen; when the duplicate thread-starter MESSAGE_CREATE
arrives, on_message's guard returns True and the event is dropped.

This is a minimal, targeted fix:
- No new state: reuses the existing _dedup instance
- No timing/race: the pre-seed happens synchronously inside the async
  _handle_message, before the thread-starter event can be dispatched
- Scoped: only fires when auto-threading is enabled AND thread creation
  succeeds (thread object is not None)

Also adds tests in tests/gateway/test_discord_double_dispatch.py
covering the pre-seed behaviour, failure modes (thread creation fails,
auto-thread disabled), and dedup cache integrity.

Closes #51057

plugins/platforms/discord/adapter.py

@@ -5285,6 +5285,16 @@ class DiscordAdapter(BasePlatformAdapter):
                     thread_id = str(thread.id)
                     auto_threaded_channel = thread
                     self._threads.mark(thread_id)
+                    # Pre-seed dedup: when _auto_create_thread creates a thread
+                    # via message.create_thread(), Discord fires a second
+                    # MESSAGE_CREATE event for the "thread starter message".
+                    # That starter message carries id == thread.id and may
+                    # arrive with type=default (not type=21/thread_starter_message),
+                    # so the type filter above does not catch it.  Marking the
+                    # thread id in the dedup cache now ensures that duplicate
+                    # event is dropped before it can trigger a second agent run.
+                    # Fixes #51057.
+                    self._dedup.is_duplicate(str(thread.id))
 
         referenced_attachments = []
         reference = getattr(message, "reference", None)