Add URI well-formed validation to mapped_pages URLs

- Enhanced validation to include Uri.IsWellFormedUriString check
- Added test cases for malformed URIs, invalid characters, and non-absolute URIs
- URLs must now be both properly prefixed AND well-formed absolute URIs

Co-authored-by: cotti <[email protected]>

Author: Copilot

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -364,7 +364,7 @@ private YamlFrontMatter ProcessYamlFrontMatter(MarkdownDocument document)
 		{
 			foreach (var url in fm.MappedPages)
 			{
-				if (!string.IsNullOrEmpty(url) && !url.StartsWith("https://www.elastic.co/guide", StringComparison.OrdinalIgnoreCase))
+				if (!string.IsNullOrEmpty(url) && (!url.StartsWith("https://www.elastic.co/guide", StringComparison.OrdinalIgnoreCase) || !Uri.IsWellFormedUriString(url, UriKind.Absolute)))
 				{
 					Collector.EmitError(FilePath, $"Invalid mapped_pages URL: \"{url}\". All mapped_pages URLs must start with \"https://www.elastic.co/guide\". Please update the URL to reference content under the Elastic documentation guide.");
 				}

tests/Elastic.Markdown.Tests/FrontMatter/YamlFrontMatterTests.cs

@@ -279,3 +279,60 @@ public void HasErrorsForExternalUrl()
 		Collector.Diagnostics.Should().Contain(d => d.Message.Contains("Invalid mapped_pages URL: \"https://github.com/elastic/docs-builder\". All mapped_pages URLs must start with \"https://www.elastic.co/guide\""));
 	}
 }
+
+public class MappedPagesMalformedUri(ITestOutputHelper output) : DirectiveTest(output,
+	"""
+	---
+	mapped_pages:
+	  - "https://www.elastic.co/guide/[invalid-characters]"
+	---
+
+	# Test Page
+	"""
+)
+{
+	[Fact]
+	public void HasErrorsForMalformedUri()
+	{
+		Collector.Diagnostics.Should().HaveCount(1);
+		Collector.Diagnostics.Should().Contain(d => d.Message.Contains("Invalid mapped_pages URL: \"https://www.elastic.co/guide/[invalid-characters]\". All mapped_pages URLs must start with \"https://www.elastic.co/guide\""));
+	}
+}
+
+public class MappedPagesInvalidScheme(ITestOutputHelper output) : DirectiveTest(output,
+	"""
+	---
+	mapped_pages:
+	  - "https://www.elastic.co/guide/invalid uri with spaces"
+	---
+
+	# Test Page
+	"""
+)
+{
+	[Fact]
+	public void HasErrorsForInvalidScheme()
+	{
+		Collector.Diagnostics.Should().HaveCount(1);
+		Collector.Diagnostics.Should().Contain(d => d.Message.Contains("Invalid mapped_pages URL: \"https://www.elastic.co/guide/invalid uri with spaces\". All mapped_pages URLs must start with \"https://www.elastic.co/guide\""));
+	}
+}
+
+public class MappedPagesNotAbsoluteUri(ITestOutputHelper output) : DirectiveTest(output,
+	"""
+	---
+	mapped_pages:
+	  - "not-a-uri-at-all"
+	---
+
+	# Test Page
+	"""
+)
+{
+	[Fact] 
+	public void HasErrorsForNotAbsoluteUri()
+	{
+		Collector.Diagnostics.Should().HaveCount(1);
+		Collector.Diagnostics.Should().Contain(d => d.Message.Contains("Invalid mapped_pages URL: \"not-a-uri-at-all\". All mapped_pages URLs must start with \"https://www.elastic.co/guide\""));
+	}
+}