When a route is registered with multiple HTTP methods via
add_api_route(..., methods=["POST", "DELETE"]), generate_unique_id()
only uses the first method from the set, causing all methods to share
the same unique_id and producing duplicate operationIds in the OpenAPI
schema along with a UserWarning.

Fix by detecting multi-method routes in get_openapi_operation_metadata
and building a per-method operationId using the current method parameter
(already available in the function), following the same pattern as
generate_unique_id.

Fixes fastapi#13175