Move session initialization from graphql_process_http_request hook to
init_graphql_request hook with an is_graphql_http_request() guard.
graphql_process_http_request fires before WPGraphQL's OPTIONS check in
Router::process_http_request(), causing wc_load_cart() to create a new
session for every preflight request. init_graphql_request fires inside
Request constructor which is only instantiated after the OPTIONS exit.
The is_graphql_http_request() guard ensures initialize_session_and_cart()
only runs for actual HTTP requests, not programmatic graphql() calls in
tests or internal usage.
