Fix user delete flow 2

tests/test_onboarding_flow.sh

@@ -845,7 +845,8 @@ log_step "6.1. Getting deletion preview for test user"
 
 # First, get a preview of what would be deleted
 DELETION_PREVIEW_RESPONSE=$(curl -s -w "\nHTTP_CODE:%{http_code}" -X GET "$API_BASE/api/v1/users/delete/$USER_ID/deletion-preview" \
-    -H "Authorization: Bearer $ACCESS_TOKEN")
+    -H "Authorization: Bearer $ACCESS_TOKEN" \
+    -H "Content-Type: application/json")
 
 # Extract HTTP code and response
 HTTP_CODE=$(echo "$DELETION_PREVIEW_RESPONSE" | grep "HTTP_CODE:" | cut -d: -f2)