Fix HTML Entities Conversion Issue (#347)

Problem

When processing HTML text containing HTML entities, such as ©, PreMailer.Net converts them to the corresponding characters (such as ©). This causes display issues in Gmail, which shows a notice that the message was trimmed when it encounters the © character, but displays correctly when using the © entity.

Root Cause

AngleSharp's HTML parser converts HTML entities to characters during parsing, and the default HtmlMarkupFormatter only re-encodes specific characters (&, <, >, ", etc.) during serialization. For other special characters like the copyright symbol (©), it doesn't convert them back to their entity form.

Solution

1. Created a custom PreserveEntitiesHtmlMarkupFormatter that extends AngleSharp's HtmlMarkupFormatter to convert special characters back to their HTML entity equivalents during serialization.
2. Added a new preserveEntities parameter to all MoveCssInline method overloads to control whether HTML entities should be preserved.
3. When preserveEntities is set to true, the custom formatter is used instead of the default one.

Testing

• Added unit tests for the custom formatter to verify that it correctly preserves HTML entities.
• Added integration tests for the MoveCssInline method to verify that the preserveEntities parameter works correctly.
• All existing tests continue to pass, ensuring no regressions were introduced.

Usage

// Preserve HTML entities like ©
var result = PreMailer.MoveCssInline(html, preserveEntities: true);

Link to Devin run

Requested by: m@martinnormark.com