fix: replace .loose() with .passthrough() in Zod schema

The .loose() method doesn't exist in Zod - replace with .passthrough()
to allow additional properties in the anthropicStreamEventSchema.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: lanej

tests/anthropic-response.test.ts

@@ -63,7 +63,7 @@ const anthropicStreamEventSchema = z
       "message_stop",
     ]),
   })
-  .loose()
+  .passthrough()
 
 function isValidAnthropicStreamEvent(payload: unknown): boolean {
   return anthropicStreamEventSchema.safeParse(payload).success