fix(bootstrap): enforce harvester_kubeconfig_path when manage_storage_class = true

Summary

The variable harvester_kubeconfig_path in modules/bootstrap/variables.tf is documented as required when manage_storage_class = true, but no enforcement exists. This causes null_resource.patch_longhorn_sc in main.tf to run with an empty KUBECONFIG environment variable, resulting in a runtime failure.

Fix

Add a check block in modules/bootstrap/main.tf to enforce the requirement (cross-variable validations are not supported in variable blocks):

check "harvester_kubeconfig_path_required" {
  assert {
    condition     = !var.manage_storage_class || var.harvester_kubeconfig_path != ""
    error_message = "harvester_kubeconfig_path is required when manage_storage_class = true. Set KUBECONFIG to a valid Harvester kubeconfig path."
  }
}

References

• PR: https://github.com/wso2/open-cloud-datacenter/pull/84
• Review comment: https://github.com/wso2/open-cloud-datacenter/pull/84#discussion_r3167420924
• Requested by: @HiranAdikari

Start in modules/bootstrap/main.tf and review the null_resource.patch_longhorn_sc behavior described in the issue, then compare it with the variable documentation in modules/bootstrap/variables.tf. Add the requested cross-variable validation and verify that storage-class management requires a non-empty kubeconfig path while the other case remains valid.