Public records — Action seeking declaration that private physician groups contracted to provide emergency room staff at facilities operated by public hospital district are subject to public records law and that groups’ business records are public records, and seeking writ of mandamus compelling production of those records — Physician groups are not agencies within meaning of Public Records Law where evidence indicates that groups have been contracted to provide select services to district in furtherance of its governmental function, not to provide services on behalf of district in satisfaction of its governmental function