fix(rss): emit UTF-8 declaration and add standard channel metadata

The RSS/Atom feeds were written through a StringBuilder, so XmlWriter stamped
the declaration as encoding="utf-16" even though OutputWriter persists the
string as UTF-8. Strict parsers (System.Xml, Python ElementTree, many feed
readers) reject the feed because the declared encoding disagrees with the
on-disk bytes.

Route the writer through a Utf8StringWriter that reports UTF-8 so the
declaration matches. Also add the standard channel-level generator, language,
and pubDate elements (parity with the mkdocs-rss feed) that readers expect.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

src/Netdocs.Plugins/RssPlugin.cs

@@ -107,7 +107,7 @@ private FeedItem ToItem(BlogPost post, string siteUrl)
     private string BuildRss(SiteContext site, string siteUrl, List<FeedItem> items)
     {
         var sb = new StringBuilder();
-        using var writer = XmlWriter.Create(sb, XmlSettings());
+        using var writer = XmlWriter.Create(new Utf8StringWriter(sb), XmlSettings());
 
         writer.WriteStartDocument();
         writer.WriteStartElement("rss");
@@ -119,8 +119,14 @@ private string BuildRss(SiteContext site, string siteUrl, List<FeedItem> items)
         writer.WriteElementString("title", _feedTitle ?? site.Config.SiteName);
         writer.WriteElementString("link", siteUrl + "/");
         writer.WriteElementString("description", _feedDescription ?? site.Config.SiteDescription ?? site.Config.SiteName);
+        if (!string.IsNullOrWhiteSpace(site.Config.Theme.Language))
+            writer.WriteElementString("language", site.Config.Theme.Language);
         if (items.Count > 0)
+        {
+            writer.WriteElementString("pubDate", items[0].Date.ToString("r"));
             writer.WriteElementString("lastBuildDate", items[0].Date.ToString("r"));
+        }
+        writer.WriteElementString("generator", "Netdocs");
         if (_ttl > 0)
             writer.WriteElementString("ttl", _ttl.ToString());
 
@@ -176,7 +182,7 @@ private string BuildRss(SiteContext site, string siteUrl, List<FeedItem> items)
     private string BuildAtom(SiteContext site, string siteUrl, List<FeedItem> items)
     {
         var sb = new StringBuilder();
-        using var writer = XmlWriter.Create(sb, XmlSettings());
+        using var writer = XmlWriter.Create(new Utf8StringWriter(sb), XmlSettings());
 
         writer.WriteStartDocument();
         writer.WriteStartElement("feed", AtomNs);
@@ -234,6 +240,18 @@ private string BuildAtom(SiteContext site, string siteUrl, List<FeedItem> items)
     private static XmlWriterSettings XmlSettings() =>
         new() { Indent = true, Encoding = new UTF8Encoding(false) };
 
+    /// <summary>
+    /// <see cref="XmlWriter"/> stamps the XML declaration with the writer's reported encoding.
+    /// The default <see cref="StringWriter"/> reports UTF-16, so the declaration ends up as
+    /// <c>encoding="utf-16"</c> even though the string is later persisted as UTF-8 — a mismatch
+    /// that makes strict RSS parsers reject the feed. Reporting UTF-8 keeps the declaration
+    /// consistent with the bytes actually written to disk.
+    /// </summary>
+    private sealed class Utf8StringWriter(StringBuilder sb) : StringWriter(sb)
+    {
+        public override Encoding Encoding => Encoding.UTF8;
+    }
+
     private static string? FrontMatterString(Page page, string key) =>
         page.FrontMatter.TryGetValue(key, out var v) ? v?.ToString() : null;
 

tests/Netdocs.Core.Tests/RssPluginTests.cs

@@ -73,6 +73,36 @@ public async Task WritesRssFeedWithItemMetadata()
         Assert.Contains("rel=\"self\"", xml);
     }
 
+    [Fact]
+    public async Task DeclaresUtf8AndParsesStrictly()
+    {
+        var site = Site(Post("Hello", "blog/hello", new DateTimeOffset(2024, 1, 2, 0, 0, 0, TimeSpan.Zero),
+            categories: new[] { "News" }));
+        await Run(new Dictionary<string, object?>(), site);
+
+        var path = Path.Combine(_root, "feed_rss_created.xml");
+        var declaration = File.ReadLines(path).First();
+        Assert.Contains("encoding=\"utf-8\"", declaration, StringComparison.OrdinalIgnoreCase);
+
+        // Parse from the raw bytes so the declared encoding is honored — this throws if the
+        // declaration disagrees with the on-disk bytes.
+        var doc = new System.Xml.XmlDocument();
+        doc.Load(path);
+        Assert.Equal("rss", doc.DocumentElement!.Name);
+    }
+
+    [Fact]
+    public async Task RssChannel_IncludesStandardMetadata()
+    {
+        var site = Site(Post("Hello", "blog/hello", new DateTimeOffset(2024, 1, 2, 0, 0, 0, TimeSpan.Zero)));
+        await Run(new Dictionary<string, object?>(), site);
+
+        var xml = File.ReadAllText(Path.Combine(_root, "feed_rss_created.xml"));
+        Assert.Contains("<generator>Netdocs</generator>", xml);
+        Assert.Contains("<language>en</language>", xml);
+        Assert.Contains("<pubDate>", xml);
+    }
+
     [Fact]
     public async Task PerPostTitleOverride_WinsOverPageTitle()
     {