fix(slack): Use set_user for email in Slack event webhook

Replace set_tag("user.email", ...) with set_user({"email": ...}) so the
user email is properly stored as sentry.user.email in span attributes,
matching the EAP search attribute definition.

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

Author: DominikB2014

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

@@ -279,7 +279,7 @@ def on_link_shared(self, request: Request, slack_request: SlackDMRequest) -> boo
             sentry_sdk.set_tag("organization.slug", organization.slug)
         identity_user = slack_request.get_identity_user()
         if identity_user:
-            sentry_sdk.set_tag("user.email", identity_user.email)
+            sentry_sdk.set_user({"email": identity_user.email})
 
         logger_params = {
             "integration_id": slack_request.integration.id,