feat(terraform_plan): `include_nested_expressions` on `direct_references`

One boolean plus a _collect_references helper. Reference edges inside nested blocks (e.g. Azure
private-endpoint private_service_connection blocks) are invisible today, blocking a whole family
of cross-resource checks (private endpoints, network associations and similar). Specify how
the flattened set plugs into references_to's counter at handler.py:461-474 (which already
double-counts — the break exits only the inner loop), and check in hand-written fixtures (the
existing suite shells out to the terraform binary). Document the cap: _normalize_reference
drops var/local/module, so module-per-service plans produce no edges at all.

Start with the _collect_references helper and handler.py:461-474, then trace how direct_references and references_to count edges. Add hand-written fixtures for nested blocks such as Azure private_endpoint private_service_connection and verify the flattened references without relying only on the Terraform-binary suite. Document the _normalize_reference limitation for var, local, and module references.