fix(slack): Cast user id to string in set_user call

sentry_sdk.set_user expects a string for the id field. Passing an int
could prevent user.id from being indexed correctly in EAP spans.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: DominikB2014

src/sentry/integrations/slack/webhooks/event.py

@@ -281,7 +281,7 @@ def on_link_shared(self, request: Request, slack_request: SlackDMRequest) -> boo
         if identity_user:
             sentry_sdk.set_user(
                 {
-                    "id": identity_user.id,
+                    "id": str(identity_user.id),
                     "email": identity_user.email,
                     "username": identity_user.username,
                 }