Fix concurrent HtmlParser corruption by removing static singleton

[Copilot]

HtmlParser was a static readonly singleton, making it shared across all YoutubeClient instances and threads. AngleSharp's HtmlParser is not thread-safe — concurrent calls corrupt its internal Dictionary state, throwing InvalidOperationException.

Changes

• YoutubeExplode/Utils/Html.cs: Replace the static HtmlParser field with a new instance per Parse() call, with a comment explaining the thread-safety rationale.

// Before
private static readonly HtmlParser HtmlParser = new();
public static IHtmlDocument Parse(string source) => HtmlParser.ParseDocument(source);

// After
public static IHtmlDocument Parse(string source) => new HtmlParser().ParseDocument(source);

This allows multiple YoutubeClient instances to be used concurrently without corrupting shared parser state.

[Copilot]

Pull request overview

Fixes thread-safety corruption in Html.Parse by replacing the shared static HtmlParser singleton with a per-call instance, since AngleSharp's HtmlParser is not thread-safe.

Changes:

• Remove static HtmlParser field in Html utility class.
• Instantiate a new HtmlParser per Parse() call with explanatory comment referencing issue #955.

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