fix(slack): Use set_user for email in Slack event webhook (#112530)

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. Previously, `user.email`
was
never populated for spans with `slack.event_type` because the tag key
didn't
match the expected internal attribute name.

Also add a few other fields to user

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: DominikB2014

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

@@ -279,7 +279,13 @@ 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(
+                {
+                    "id": identity_user.id,
+                    "email": identity_user.email,
+                    "username": identity_user.username,
+                }
+            )
 
         logger_params = {
             "integration_id": slack_request.integration.id,