Allow button directives to use Markdown link references (#3443)

* Initial plan

* fix: allow button directives to use link references

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Fabrizio Ferri-Benedetti <fabri.ferribenedetti@elastic.co>

Author: Copilot

src/Elastic.Markdown/Myst/Directives/Button/ButtonBlock.cs

@@ -56,8 +56,8 @@ public partial class ButtonBlock(DirectiveBlockParser parser, ParserContext cont
 	/// </summary>
 	public bool IsInGroup { get; private set; }
 
-	// Regex to match a Markdown link: [text](url)
-	[GeneratedRegex(@"^\s*\[[^\]]+\]\([^\)]+\)\s*$", RegexOptions.Singleline)]
+	// Regex to match a single Markdown link, including reference-style links.
+	[GeneratedRegex(@"^\s*\[[^\]]*\](?:\([^\)]+\)|\[[^\]]+\]|\[\])\s*$", RegexOptions.Singleline)]
 	private static partial Regex LinkPattern();
 
 	public override void FinalizeAndValidate(ParserContext context)
@@ -94,14 +94,14 @@ private void ValidateContent()
 
 		if (string.IsNullOrWhiteSpace(content))
 		{
-			this.EmitError("Button directive requires a link. Use: :::{button}\n[text](url)\n:::");
+			this.EmitError("Button directive requires a link. Use: :::{button}\n[text](url)\n:::\nOr: :::{button}\n[text][ref]\n:::");
 			return;
 		}
 
 		// Check if content matches the link pattern
 		if (!LinkPattern().IsMatch(content))
 		{
-			this.EmitError("Button directive must contain only a single Markdown link. Use: :::{button}\n[text](url)\n:::");
+			this.EmitError("Button directive must contain only a single Markdown link. Use: :::{button}\n[text](url)\n:::\nOr: :::{button}\n[text][ref]\n:::");
 		}
 	}
 

tests/Elastic.Markdown.Tests/Directives/ButtonTests.cs

@@ -81,6 +81,26 @@ public class ButtonExternalTests(ITestOutputHelper output) : DirectiveTest<Butto
 	public void RendersNoopenerNoreferrer() => Html.Should().Contain("rel=\"noopener noreferrer\"");
 }
 
+public class ButtonReferenceLinkTests(ITestOutputHelper output) : DirectiveTest<ButtonBlock>(output,
+"""
+:::{button}
+[Open][kibana-url]
+:::
+
+[kibana-url]: <https://foo.example.com>
+"""
+)
+{
+	[Fact]
+	public void RendersReferencedLinkHref() => Html.Should().Contain("href=\"https://foo.example.com\"");
+
+	[Fact]
+	public void RendersButtonText() => Html.Should().Contain(">Open<");
+
+	[Fact]
+	public void EmitsNoErrors() => Collector.Diagnostics.Should().BeEmpty();
+}
+
 public class ButtonInvalidTypeTests(ITestOutputHelper output) : DirectiveTest<ButtonBlock>(output,
 """
 :::{button}