fix(autofix): Add missing @EventClass decorator to AiAutofixPhaseEvent base class

The base event class was missing the @analytics.eventclass() decorator,
which meant it wasn't processed as a proper dataclass. Child classes
couldn't accept keyword arguments like organization_id.

Agent transcript: https://claudescope.sentry.dev/share/0U9SUOfdYVu5539zSG_tdufbBsNcV98mrxSXz9fNeIA

Author: chromy

src/sentry/analytics/events/autofix_events.py

@@ -1,6 +1,7 @@
 from sentry import analytics
 
 
+@analytics.eventclass()
 class AiAutofixPhaseEvent(analytics.Event):
     organization_id: int
     project_id: int