fix(workload-identity): resolve inconsistent conditional result types in output

When use_existing_gcp_sa is unknown at plan time, Terraform fails with
"Inconsistent conditional result types" because the data source and resource
have slightly different object schemas. Replace the ternary with
one(concat(...)) so Terraform never needs to type-check across the two
branches — only one list ever has elements based on their mutually exclusive
count conditions.

Fixes #1112

Author: raman1236

modules/workload-identity/output.tf

@@ -41,5 +41,8 @@ output "gcp_service_account_name" {
 
 output "gcp_service_account" {
   description = "GCP service account."
-  value       = var.use_existing_gcp_sa ? data.google_service_account.cluster_service_account[0] : google_service_account.cluster_service_account[0]
+  value = one(concat(
+    data.google_service_account.cluster_service_account,
+    google_service_account.cluster_service_account
+  ))
 }