Fix sub-path resolution in DiagnosticLinkInlineParser (#884)

* Fix sub-path resolution in DiagnosticLinkInlineParser

Ensure sub-prefix calculations handle edge cases where lastIndexOf returns -1. This prevents potential runtime errors and ensures correct path resolution for sibling links.

Add `elasticsearch` to our smoke tests since it's a high traffic repository.

* Update landing-page-path for elastic/elasticsearch

Corrected the landing-page-path to ensure consistent trailing slash usage. This aligns with the paths of other repositories and prevents potential inconsistencies in URL handling.

* off by one :sadpanda:

Author: Mpdreamz

.github/workflows/smoke-test.yml

@@ -13,6 +13,8 @@ jobs:
         include:
           - repository: elastic/docs-content
             landing-page-path-output: /docs/
+          - repository: elastic/elasticsearch
+            landing-page-path-output: /docs/reference/
           - repository: elastic/apm-agent-android
             landing-page-path-output: /docs/reference/
 

src/Elastic.Markdown/Myst/InlineParsers/DiagnosticLinkInlineParser.cs

@@ -309,8 +309,9 @@ private static void UpdateLinkUrl(LinkInline link, string url, ParserContext con
 			// './' current path lookups should be relative to sub-path.
 			// If it's not e.g /reference/cloud-k8s/api-docs/ these links should resolve on folder up.
 			var siblingsGoToCurrent = url.StartsWith("./") && markdownPath == "index.md";
-			if (!siblingsGoToCurrent)
-				subPrefix = subPrefix[..subPrefix.LastIndexOf('/')];
+			var lastIndexPath = subPrefix.LastIndexOf('/');
+			if (lastIndexPath >= 0 && !siblingsGoToCurrent)
+				subPrefix = subPrefix[..lastIndexPath];
 
 			var combined = '/' + Path.Combine(subPrefix, url).TrimStart('/');
 			url = Path.GetFullPath(combined);