fix: prevent duplicate messages — gateway dedup + partial stream guard (#4878)

* fix(gateway): add message deduplication to Discord and Slack adapters (#4777)

Discord RESUME replays events after reconnects (~7/day observed),
and Slack Socket Mode can redeliver events if the ack was lost.
Neither adapter tracked which messages were already processed,
causing duplicate bot responses.

Add _seen_messages dedup cache (message ID → timestamp) with 5-min
TTL and 2000-entry cap to both adapters, matching the pattern already
used by Mattermost, Matrix, WeCom, Feishu, DingTalk, and Email.

The check goes at the very top of the message handler, before any
other logic, so replayed events are silently dropped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: prevent duplicate messages on partial stream delivery

When streaming fails after tokens are already delivered to the platform,
_interruptible_streaming_api_call re-raised the error into the outer
retry loop, which would make a new API call — creating a duplicate
message.

Now checks deltas_were_sent before re-raising: if partial content was
already streamed, returns a stub response instead. The outer loop treats
the turn as complete (no retry, no fallback, no duplicate).

Inspired by PR #4871 (@trevorgordon981) which identified the bug.
This implementation avoids monkey-patching exception objects and keeps
the fix within the streaming call boundary.

---------

Co-authored-by: Mibayy <mibayy@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gateway/platforms/discord.py

@@ -449,6 +449,11 @@ class DiscordAdapter(BasePlatformAdapter):
         self._bot_task: Optional[asyncio.Task] = None
         # Cap to prevent unbounded growth (Discord threads get archived).
         self._MAX_TRACKED_THREADS = 500
+        # Dedup cache: message_id → timestamp.  Prevents duplicate bot
+        # responses when Discord RESUME replays events after reconnects.
+        self._seen_messages: Dict[str, float] = {}
+        self._SEEN_TTL = 300   # 5 minutes
+        self._SEEN_MAX = 2000  # prune threshold
 
     async def connect(self) -> bool:
         """Connect to Discord and start receiving events."""
@@ -539,6 +544,19 @@ class DiscordAdapter(BasePlatformAdapter):
 
             @self._client.event
             async def on_message(message: DiscordMessage):
+                # Dedup: Discord RESUME replays events after reconnects (#4777)
+                msg_id = str(message.id)
+                now = time.time()
+                if msg_id in adapter_self._seen_messages:
+                    return
+                adapter_self._seen_messages[msg_id] = now
+                if len(adapter_self._seen_messages) > adapter_self._SEEN_MAX:
+                    cutoff = now - adapter_self._SEEN_TTL
+                    adapter_self._seen_messages = {
+                        k: v for k, v in adapter_self._seen_messages.items()
+                        if v > cutoff
+                    }
+
                 # Always ignore our own messages
                 if message.author == self._client.user:
                     return