fix(slack): close previous handler in connect() to prevent zombie Socket Mode connections

SlackAdapter.connect() overwrote self._handler, self._app, and
self._socket_mode_task without closing the prior AsyncSocketModeHandler
first. If connect() was called a second time on the same adapter (e.g.
during a gateway restart or in-process reconnect attempt), the old Socket
Mode websocket stayed alive. Both the old and new connections received
every Slack event and dispatched it twice — producing double responses
with different wording, the same bug that affected DiscordAdapter (#18187,
fixed in #18758).

Fix: add a close-before-reassign guard at the start of the connection
setup path, mirroring the guard DiscordAdapter.connect() already has.
When self._handler is None (fresh adapter, first connect()) the block is
a harmless no-op. Scoped to the handler/app fields only — no behavior
change for any path that does not call connect() twice.

Fixes #18980

gateway/platforms/slack.py

@@ -528,6 +528,21 @@ class SlackAdapter(BasePlatformAdapter):
                 return False
             lock_acquired = True
 
+            # Close any previous handler before creating a new one so that
+            # calling connect() a second time (e.g. during a gateway restart or
+            # in-process reconnect attempt) does not leave a zombie Socket Mode
+            # connection alive.  Both the old and new connections would otherwise
+            # receive every Slack event and dispatch it twice, producing double
+            # responses — the same bug that affected DiscordAdapter (#18187).
+            if self._handler is not None:
+                try:
+                    await self._handler.close_async()
+                except Exception:
+                    logger.debug("[%s] Failed to close previous Slack handler", self.name)
+                finally:
+                    self._handler = None
+                    self._app = None
+
             # First token is the primary — used for AsyncApp / Socket Mode
             primary_token = bot_tokens[0]
             self._app = AsyncApp(token=primary_token)