fix(page-card): apply UrlPathPrefix to resolved URLs

Page-card links were generating site-root-relative hrefs (e.g. /cli/stack/es)
without the deployment path prefix, causing 404s in preview environments where
the site is hosted under a subpath (e.g. /elastic/cli/pull/406/cli/stack/es).

Apply the same UrlPathPrefix logic used by DiagnosticLinkInlineParser.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: theletterf

src/Elastic.Markdown/Myst/Directives/PageCard/PageCardBlock.cs

@@ -55,6 +55,11 @@ public override void FinalizeAndValidate(ParserContext context)
 			: relativeToSource;
 
 		ResolvedUrl = "/" + withoutExtension.Replace('\\', '/');
+
+		// Apply URL path prefix so links work in preview/sub-path deployments (same logic as DiagnosticLinkInlineParser)
+		var urlPathPrefix = context.Build.UrlPathPrefix ?? string.Empty;
+		if (!string.IsNullOrWhiteSpace(urlPathPrefix) && !ResolvedUrl.StartsWith(urlPathPrefix, StringComparison.OrdinalIgnoreCase))
+			ResolvedUrl = $"{urlPathPrefix.TrimEnd('/')}{ResolvedUrl}";
 	}
 
 	[GeneratedRegex(@"^\[([^\]]+)\]\(([^)]+)\)$")]