- #3918
- lazy-load `TagBuilder.InnerHtml`; add `HasInnerHtml` property for conservative use
- depends on aspnet/HtmlAbstractions@0781b5a (adds `HtmlContentBuilder.Count`)
- don't use a `HtmlContentBuilder` at all in `DefaultHtmlGenerator.GenerateValidationSummary()`
- avoid instantiating `HtmlContentBuilder` when short-circuits make it unnecessary
- provide correct `HtmlContentBuilder` capacity where known
