Add reverse HTML parser for WebExpress.WebCore.WebHtml

[Copilot]

Implements the inverse of the existing HTML renderer: a parser that takes an arbitrary HTML string and reconstructs the corresponding IHtmlNode object tree from the WebExpress.WebCore.WebHtml namespace.

Core components (WebExpress.WebCore/WebHtml/Parser/)

• HtmlTokenType – enum of 7 token categories (Doctype, StartTag, EndTag, SelfClosingTag, Text, Comment, EndOfFile)
• HtmlTokenAttribute – per-attribute model; Value == null signals boolean attributes (disabled, checked, etc.)
• HtmlToken – immutable token produced by the tokenizer
• HtmlTokenizer – lenient character-level tokenizer; handles void elements, quoted/unquoted/boolean attributes, comments, DOCTYPE, and recovers stray < as text
• HtmlElementFactory – case-insensitive registry mapping all known tag names to their specific HtmlElement subclass; unknown tags fall back to new HtmlElement(tagName) for robustness. Accepts both kbd (correct HTML) and kdb (mirrors existing class-name typo)
• HtmlParser – recursive descent parser; tolerates unclosed tags and produces a flat IReadOnlyList<IHtmlNode> or single-root variant
• HtmlParseException – exception with optional Position property, consistent with SocketHandshakeException style

Usage

var parser = new HtmlParser();

// Returns IReadOnlyList<IHtmlNode>
var nodes = parser.Parse("<div class=\"container\"><p id=\"intro\">Hello</p></div>");

var div   = nodes.OfType<HtmlElementTextContentDiv>().Single();   // HtmlElementTextContentDiv
var p     = div.Elements.OfType<HtmlElementTextContentP>().Single();
var text  = p.Elements.OfType<HtmlText>().Single();               // "Hello"

// Round-trip: render → parse → render produces equivalent HTML
var img = new HtmlElementMultimediaImg { Src = "logo.png", Alt = "Logo" };
var restored = parser.Parse(img.ToString().Trim())
                     .OfType<HtmlElementMultimediaImg>().Single();
Assert.Equal(img.ToString(), restored.ToString());

Tests (WebExpress.WebCore.Test/Html/Parser/)

47 new tests across three classes:

• UnitTestHtmlTokenizer – tokenizer unit tests (empty input, tag types, attribute variants, comments, DOCTYPE, name normalisation)
• UnitTestHtmlElementFactory – factory registration, case-insensitivity, unknown tags, null guard
• UnitTestHtmlParser – element reconstruction, nested/deep structures, text nodes, self-closing/void tags, boolean/data/ARIA attributes, comments, DOCTYPE, unknown tags, malformed HTML, and round-trip correctness