Use BCL values for ASP.NET cache expiration sentinels

Replace resolved value reads of System.Web.Caching.Cache.NoAbsoluteExpiration/NoSlidingExpiration with their exact DateTime.MaxValue/TimeSpan.Zero initializers. Preserve DateTime kind and ticks. Leave nameof, expression trees, by-reference contexts, arbitrary call/constructor arguments, unresolved/lookalike fields and conditional snapshots unchanged. Cache.Add/Insert arguments are supported because their signatures consume values. No cache implementation or expiration policy is selected. Remove unused top-level legacy imports conservatively.