chore: add changeset for RunEvent schema & ApiClient (triggerdotdev#3220)

Author: deepshekhardas

.changeset/runevent-schema-apiclient.md

@@ -0,0 +1,7 @@
+---
+"@trigger.dev/core": patch
+---
+
+fix(core): define RunEvent schema and update ApiClient validation
+
+Defines proper RunEvent schema with Zod validation and updates ApiClient to use the new schema for improved type safety and runtime validation.