Fix TestSet fileCount field to be nullable

- Changed fileCount from Int! to Int in TestSet type
- Resolves GraphQL validation error when backend returns null fileCount
- Allows test sets with unknown file counts during processing

Author: Taniya Mathur

src/api/schema.graphql

@@ -303,7 +303,7 @@ type TestSet @aws_cognito_user_pools @aws_iam {
   id: String!
   name: String!
   filePattern: String
-  fileCount: Int!
+  fileCount: Int
   status: String
   createdAt: AWSDateTime!
   error: String