Fix CS8981: Rename lowercase type names to PascalCase

[Copilot]

The C# compiler emits CS8981 warnings for type names using only lowercase ASCII characters (changefreq, url, urlset), as these may become reserved keywords in future language versions.

Changes

• Renamed types to PascalCase:

  • changefreq → ChangeFreq
  • url → Url
  • urlset → UrlSet
  • urlCollection → UrlCollection

• Preserved XML serialization compatibility by explicitly specifying lowercase names in attributes:

  [XmlTypeAttribute("changefreq", Namespace = "...")]
  [XmlRootAttribute("changefreq", Namespace = "...")]
  public enum ChangeFreq

• Updated all references in SiteManager.cs, ISiteManager.cs, and test files

This maintains backward compatibility with existing XML sitemap files while eliminating all 6 CS8981 warnings.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix CS8981: Type names use only lower-cased ascii characters (may become reserved)</issue_title>
<issue_description>The following type names use only lower-cased ascii characters, which may become reserved for the language:

• changefreq (changefreq.cs, GoogleSiteMap.cs)
• urlset (GoogleSiteMap.cs, urlset.cs)
• url (GoogleSiteMap.cs, url.cs)

Consider renaming these types to avoid future compatibility issues.

Files:

• DasBlog.Services\Seo\GoogleSiteMap\changefreq.cs
• DasBlog.Services\Seo\GoogleSiteMap\GoogleSiteMap.cs
• DasBlog.Services\Seo\GoogleSiteMap\url.cs
• DasBlog.Services\Seo\GoogleSiteMap\urlset.cs</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix CS8981: Type names use only lower-cased ascii characters (may become reserved) #738

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